Conspicuously Selective Perception

A cousin of The Guards Must Be Crazy. In many games, particularly those involving stealth, enemies will often be overly sensitive towards noises that are made by the player character directly, such as footsteps, but pay no attention to noises made by NPCs or by machinery the player has set in motion. In addition, NPCs will often pay no attention to alterations made to the game world by the player, such as things not being in their place or a patrolling guard suddenly not walking by a sentry's post anymore.

If the player can disguise himself or otherwise be unknown to the enemies so as to be allowed to pass, they will instantly know that it's the player as soon as he does something vaguely suspicious. And by "suspicious", we mean anything the developers want to hinder the player with, such as running or having a weapon in hand. The guards will happily ignore all the NPCs running around like headless chickens but if the player does it, he might as well have "spy" tattooed to his forehead.

While there are certainly technical considerations (dealing with the large range of triggers, responses, responses to responses, etc) that can cause this, there is also the simple question of whether it would be fun. After all, a Stealth Based Game lives on predictability, and dying because a random NPC happened to walk along and screw up the guard's pattern isn't fun.

If this trope is overused in a game, it becomes blatantly obvious to the player that the game world revolves solely around their character and their actions. That the enemies know who you are, where you are and what you're doing, and are just waiting for some excuse to attack so that you'd hopefully be less likely to notice their All Seeing AI.

The worst examples of this are when you can shoot an enemy from a distance and they do not react; you're outside their designated sight/hearing range so they just keep taking hits until they die...then a few minutes later another guard comes by, takes one look at the corpse, and suddenly alarms are screaming and every guard in the level is shooting you on sight, because they just know it was you.

Video Games
"Aide: President Madagascar, somebody just sneezed in Brazil! President: SHUT . . . DOWN . . . EVERYTHING!"
 * The game Assassin's Creed is practically based around this trope. Anti-hero Altair goes everywhere wearing his distinctive white robe with red ribbons and various weapons prominently displayed. In spite of this, he can fool people into thinking he is a harmless monk merely by adopting their typical posture. Even if guards are alerted to him, as long as he manages to break line of sight and hide for a bit they will completely forget what he looks like. If he walks his horse very slowly past guards they will never realise he is an assassin, yet if he takes his mount into a trot or gallop, they will immediately realise he is a bad guy. On the flip side, the streets are filled with idiots who for some reason are eager to shove Altair about (and only him, never NPCs) and absurdly persistent beggar women who for some reason run after Altair even if he goes around pretending he is a monk.
 * Some of this is kind of justified within the plot, seeing as the Player Character is actually immersed in a computer simulation that allows him to explore his Genetic Memory. For example, the guards never learn what Altair looks like because the real Altair didn't get caught.
 * Averted neatly in almost every way in the sequel. A: Ezio's costume, while still kind of loud, is acceptable because his cape hides his weaponry and the game takes place in Renaissance Italy, where colorful clothes were acceptable. B: Beggars were replaced by Minstrels, they're still just as annoying, but there's several methods of getting rid of them, one of them being a good hard punch to the nose. C: Perform too many crimes or a main storyline Assassination, and you will become "Notorious". Guards will recognize you on sight.
 * The Thief games have many examples of this: guards react to the players footsteps, but not to the very noisy elevator the player rides on, even when it is the middle of the night and the elevator running should be unexpected. Drop a platter accidentally picked up onto the same kitchen counter it had been on and people in another room immediately shouts "Help! Thief!", knowing instantly it was you and not some random kitchen noise. In the third game, this went even further, as NPCs who found NPCs killed by other NPCs would attribute the foul deed to the player. Assassins that hunted the player would patrol the streets. Every time they bumped into an NPC guard or civilian they would announce that they had found you and promptly attack said NPC. On the other end of the spectrum, you could whack the last man in a three-man patrol, dump the body in a nearby alley, wait for the other two to come around again, knock the one in behind, drag him off, wait for the leader to come around on his own, oblivious to the fact that they're even missing ...
 * This was dealt with in the third Thief game, Deadly Shadows. Guards would react to a huge number of things compared to the original two. A door left open, an item missing ( stolen ) from somewhere, a light you put out earlier.. if they come by such things on their patrols, they get suspicious. One such discovery they may shrug off unless it's important, two they'll have a quick look. If they're already suspicious, they'll then go searching, and if they get clues beyond that, they'll raise a riot. This also applies to not meeting guards on their patrol routes. More than this, NPCs will run and find someone to help them if they catch you, often resulting in several guards gathering and going after you, alerting other guards along the way. All this also applies to most sounds, environmental or otherwise - including sometimes ones that you Don't make - and even if they do eventually give up looking for you, their suspicion level won't ever reset back to blissful ignorance.
 * Most of these reactions were also present in the original two games, more so in Thief 2 than in Thief 1: guards would notice certain doors left open (but not all), certain important items missing, doused torches and gas lamps and rope arrows or lit flares left lying around. Unarmed NPCs would run to fetch guards, and swordsmen who couldn't reach the player would run to get archers. They would also never get back to their lowest alert after they'd been raised to the highest.
 * Guards in Metal Gear Solid and its subsequent games had a notorious amount of Conspicuously Selective Perception. Seeing footsteps in the snow in an area populated by several dozen guards immediately warns the officer in question of an unwanted entity in the vicinity. However, if a guard finds another one passed out, lying face down on the floor, he'll simply kick them to wake them up and continue on. In fact, if a guard finds another one dead from a bullet wound to the head, he'll alert the other guards, but soon enough he'll forget about it and chalk it up to coincidence. A guard can hear you shoot a gun, but if you hide quick enough, he'll turn the corner, see no one is there, decide his mind was playing tricks on him, and resume his patrol. If you're quick enough and stay out of his (rigidly defined) field of vision, you can punch the daylights out of a guard, flip him over your shoulder, or even shoot him with a silenced pistol without making him do more than glance around nervously for a second. Naturally, in the game, there are Hand Wave reasons for these. Parody fanwork webcomic The Last Days of Foxhound spoofed this mercilessly.
 * Justified in one instance in Metal Gear Solid 2, in which a guard can be seen dancing up and down a hallway. If you get close enough, you can hear the tinny noise of headphones. As a result, he is oblivious to everything short of you walking up in front of him and socking him in the jaw.
 * In Metal Gear Solid: Portable Ops, the guards border on psychic at times. In the game, you can often play as a soldier whose equipment and uniform is identical to the other face-masked guards in a given mission. But they hear anything out of the ordinary, such as an explosion or gunshot, they immediately know you were responsible, even if nobody could have possibly seen you do it.
 * Then there's the infamous cardboard box trick, wherein Snake hides in an upturned cardboard box which no guards think to check unless they actually see it moving, no matter where it is? They will kick it out of the way if it's in the middle of their patrol route in the later games, but not MGS 1.
 * In Metal Gear 2: Solid Snake, the soldiers pretty much ALWAYS find cardboard boxes you're hiding in suspicious, and they check if someone's inside by shooting them, which renders the trick pretty useless. Also, they sometimes walk right on them, which hurts you, but somehow they still don't notice there's someone inside.
 * In Metal Gear Solid 4, much of the early game takes place in a various warzones. It is possible to disguise yourself as a rebel fighter during these sequences. However, if a PMC soldier notices Snake, they will immediately go full alert and ignore the other dozen or so rebels attacking them to hunt you down.
 * Remember who the PMCs are ultimately working for though and it's not much of a surprise that they would consider Snake a bigger target then some random rebel.
 * In Golden Eye 1997, guards will not react to the "pew-pew" of a silenced gun even if it originates from inches away from them. They will also not notice you shooting their helmets off their heads.
 * Also, in missions where you have to shoot out all the security cameras, nobody notices all the screens fill with static, one by one.
 * In Half-Life 2, one level is populated by creatures called antlions who go crazy and attack anything that moves on sand... provided that it's human. Explosions and moving objects have no effect. True, footsteps sound different from dragged objects and explosions, but an injured NPC attracts their attention by sitting up. This was spoofed in Concerned, where the protagonist nailed wooden planks to his feet to walk through the level undetected.
 * Which is essentially how you go about avoiding the antlions in the game. All it takes is two planks and the gravity gun and voilá: a safe route to wherever you want to go in the antlion-infested desert.
 * 47, the titular character in the Hitman games, has a rather striking appearance: he is tall, muscular, bald, very pale, has ice-blue eyes and a rather obvious barcode on the back of his head. Yet he is able to fool just about anyone, for a while at least, by taking the clothes from one of his victims and using it as a disguise. For example, he is able to pass himself off as a Chinese Triad member simply by wearing a certain costume, without having to do anything to disguise his very un-Chinese physical features. Additionally, guards will not notice the bloodstains and bulletholes where their colleagues used to stand. Despite this, if guards hear him running in a corridor adjacent to them, they will somehow know that he is an assassin and go into full alert mode. Likewise, if they see him running while in costume they will somehow know that he shouldn't be there.
 * The problem of often not being the same race is Hand Waved by the way of the fact that 47 is a multiracial clone, made up of a Columbian, a Chinese, and two Europeans. The fact that, say, a full Chinese looks different from a quarter Chinese is Hand Waved again, by saying that at a distance, 47 looks enough like the race he's supposed to be that nobody will really pay attention unless you get close, in which case your suspicion meter starts to rise.
 * Not exactly an example of this trope, but worth noting: in one of the missions of Hitman 2: Silent Assassin, 47 faces ninjas who are apparently psychic: they can see him even if he is wearing white clothes and standing on snow 50 feet away in the middle of a blizzard. If you disguise yourself as a ninja, they'll of course notice you, run up to you and begin checking your ID... which they can do even if you run away as soon as they notice you. (They just walk up to where you've been standing and wave their hands in the air a bit before announcing you're an intruder.)
 * The first game in the series has the worst guard AI, being completely unable to make basic logical conclusions. You can kill a guard and stand next to his body when someone comes to investigate but as long as you don't have your gun out they won't be the least suspicious. You can actually climb into a guard tower, take the guard out, and climb back down, all in full view of his buddies on the ground without raising any suspicion to you. Changing outfits completely removes suspicion, even if it's the same kind of outfit you just wore, even if it doesn't cover your head in any way.
 * Standing over the body is vaguely Handwaved in the manual by the mention of remaining unseen at the time of the killing, and returning to the scene as an innocuous innocent bystander.
 * In Blood Money, guards actually do notice bloodstains, but are still unable to open dumpsters and find stashed away bodies, and changing clothes still lowers significantly suspicion. Unless you run.
 * In ALL of the Hitman games, police and bodyguards ventilate 47 the moment he pulls a gun (unless he's disguised as one of them), yet they completely fail to react to other trespassing NPCs who are waving guns around openly.
 * I.E., in Hitman 2: Silent Assassin, one mission involves both 47 and a Spetznaz agent infiltrating a party at the German embassy. If the Spetznaz agent spots 47, he'll start chasing him with a pistol, and the German guards will automatically side with him against 47 despite the fact he's not supposed to be there either.
 * Several levels of Hitman 2 involve disguising 47 as a soldier. The Russian army apparently has a very high standard of discipline, as the penalty for carrying the wrong gun, not carrying a gun, entering the officers' quarters, or running in the street is in each case death.
 * Hitman: Blood Money has a Mardi Gras level where 47 is trying to kill 3 hitmen who are attempting to assassinate the governor. These hitmen are dressed in bizarre bird costumes and carrying rather large weapons, yet the cops completely fail to react if they spot these gun-toting birds chasing an apparently unarmed 47 through the streets.
 * In the game Crysis you can wander right past enemies while in stealth mode, and they won't notice you despite the large shadow you cast on the ground.
 * Shadows, nothing. They usually miss the player, even though the stealth effect is easily visible without being right on top of someone using it. Furthermore, a laser pointer on a gun will give off an obvious beam several feet long originating from the gun before it fades out and becomes the dot on whatever its pointing at, even if you're cloaked. While these things are like flares in multiplayer, enemies in single player won't ever notice.
 * Strangely, they do notice if you have a flashlight attached to your firearm instead while you're cloaked.
 * Don't forget helicopters, they (or is it pilots?) just know your exact location, even if the player run away while being in stealth mode.
 * Early in The Legend of Zelda the Wind Waker, Link must sneak around the Forbidden Fortress without his sword, as being spotted means capture. If, however, he hides in a barrel, he can't be detected unless the barrel is seen moving. This is true even if the barrel blocks the Moblin's patrol path; he stops, seems to sniff (sometimes, thanks to lack of collision detection, sticking his nose in the barrel), sometimes looks around, but then goes on his way. Justified by Moblins explicitly being really, really stupid.
 * The sequel, The Legend of Zelda Phantom Hourglass, has a few places where Link has to sneak around. The main dungeon, in particular, is full of "Phantoms", invincible (until the end) guards that chase Link as soon as he enters their line of sight or runs on one type of floor - but have no reaction beyond brief puzzlement to being hit in the back by a grappling hook that snags whatever they're holding. Justified by Phantoms being single-minded magical guardians.
 * Sly Cooper might be one of the worst cases of this. For the most part it's decent, with thugs going to inspect anything slightly suspicious. However, they completely ignore any impact to the stage. Probably the worst case of this is in Sly 2, when you are in Canada. Nobody seems to notice you bouncing the laser saw's beam all across the canyon.
 * In Sly 3, the guards in the first level don't seem to think anything is amiss about a guard who is a different species than any of them, plus is speaking with what Bentley describes as "the worst Italian accent I've ever heard!" and is unaware of one guard's nickname and reputation amongst the local mafia. To their credit however, they will demand that Sly recite a random password when spotting him, and his cover will be blown should he botch it up.
 * In the second level, the character Guru is able to transform himself into random inanimate objects (a rock, dead bush, pile of apples, etc.) to escape from pursuing guards, who will immediately "lose" Guru and give up chasing him, even if he pulls this trick right in front of them.
 * Also irritatingly averted in the second level. A helicopter is flying around the area randomly dropping bombs. Despite the fact that it appears to be doing this constantly, any enemy nearby will respond to the explosion as if it were a suspicious noise.
 * A typical example is in Jagged Alliance 2. If the guards don't see you or hear you (because you're sneaking around, it's dark, and you've got better night vision equipment...) they have no idea that anything is wrong. If a guard does see you, then provided you kill him silently, and without any of his colleagues seeing, before he can radio it in, his colleagues still have no idea that anything is wrong. And if those same colleagues' patrol then takes them right up to the still-warm corpse of their good buddy, complete with multiple bullet wounds or a throwing knife embedded in his throat... well, they still have no idea anything is wrong.
 * Don't even get me started on Strife: when you're walking around in the town, the guards won't be at all concerned by the fact that you're carrying a whole arsenal of weapons which were obviously stolen. However, if you dare to fire any of those weapons (without necessarily hitting anybody), you'll immediately set off the alarm and have the guards going atfer you. ...Unless you use a knife or shoot a poison bolt from a crossbow (both those attacks are silent). These attacks are completely stealthy, and even if the guards see you doing them, they won't care. Even if you kill their mate right in front of them.
 * In the original Deus Ex, you could shoot tranquilizer darts at your enemies. If you hit one in a group of others, they would all run around on high alert, until the one you hit falls down unconscious, at which point his friends will resume standing around, next to his unconscious body, and talk about how it must have been nothing.
 * Not to mention that if you run around, guards will hear your footsteps and start frantically searching for you, but if you cause other people to run around, even if no guard sees/hears you (since the only way to get people to run around is to attack someone in their vicinity), the guards won't care. Apparently they can tell the sound of your footstep from everyone else's.
 * Dropping or moving items, or moving around in any way other than a crouch within the earshot of enemies will make them suspicious. Allies, on the other hand, will greet you even if you're invisible. Enemies will also lose interest in your presence after about thirty seconds as long as you're out of sight - and don't even notice dead bodies or blood splattered on the walls.
 * A couple of techniques to get by enemies stealthily use this trope rather ridiculously. The first involves crouching behind boxes and pushing them forward - so long as you are hidden behind the box and move quietly, enemies won't react. The second method is to throw items or shoot darts at a spot you want the guards to investigate and face. Even if the guards see items mid-flight, they focus only on the final position of the dart/item.
 * In all of the Grand Theft Auto games, the cops are after YOU. You could be frantically trying to evade an entire motorcade of gun-toting maniacs shooting assault rifles downtown, they won't get chased. However, if you fire back, the fuzz will be over you like stink on cheese. In the same vein, NPC streetracers can crash into police cruiser at mach 2 then skid into a cartload of cancer puppies, nope, no reaction. Race on, dude! But for you, the slightest of bumps while passing them by? That's a wanted star, buddy.
 * In some of the games, random civilians can get chased by cops. However, if you get a wanted level, they will instantly forget about everything else and chase you.
 * San Andreas lets you grab a tow truck and tow a copcar away. With cops inside. And then drive it through the airport, onto the runway, and then over the edge into the bay. Fun.
 * The GTA games are pretty bad with this the other way, too. Say you've just gunned down the Las Venturas Strip with a minigun. The police would now like to speak to you. How to evade the choppers and cruisers and cops coming out of literally nowhere to shoot at you? Well, you can just drive into a Pay 'n' Spray, spend some cash, and drive back out and be law abiding for a while, and the cops won't mind. Even if they see you ride in on a motorcycle that gives them full view of you. Or say you're just near your house... just dive in and put on some sunglasses or something. Or sleep for 6 hours and the cops will be gone in the morning, even if they ran in after you and were in the process of gunning you down as you went to bed.
 * In Grand Theft Auto IV Pay 'n' Sprays no longer work if the cops see you go in (making them mostly useless). You can still "sleep them off," though. They'll also forget all about you if you get out of their search area and stay out of sight for a while.
 * On the other hand, you can cut your hair, grow a beard, change all your clothes and swap your motorcycle to a fire truck and sure enough all the enemy gang members will still instantly know it's you and shoot on sight.
 * Postal 2 is quite similar to the GTA IV example (and predated it by a number of years). Cops and armed civilians will consider anyone, not just the player, a threat if they're seen brandishing a weapon. This can lead to massive shootouts if the player kills someone then hides; civilians will pull out their guns in response to the threat, then suddenly see all the other armed civilians around, consider them a threat, and pretty soon everyone is shooting at everyone else.
 * In Avatar: The Burning Earth there are levels which require the characters to hide inside barrels and sneak into enemy locations. Guards will instantly notice if a barrel moves in their line of sight, but will completely ignore barrels which move behind their back. The worst offense is when you follow a guard - if you stop before he turns around, the guard will think nothing of the fact that the barrel which used to be on the other side of field is now right in front of him - even going so far as to walk around the barrel to get back on his appointed path!
 * Due to how the Doom engine works, monsters can be alerted by the sound of the player firing a weapon. The Chainsaw, on the other hand, makes a continuous sputtering noise for as long as it's the active weapon, but enemies won't be alerted until you actually spin the chain. The Fists work in the same way, even though there's no sound effect when punching.
 * The Flash game Pandemic II is a variation: You play as an infectious disease, and the goal of the game is to infect and kill as much of the world's population as possible. Humanity will try to thwart you by cutting off your paths of infection, such as borders, planes, and boats. However, it is entirely possible to go through most of the game with a disease with entirely mundane symptoms, such as sneezing. This has led to odd situations when countries will completely isolate themselves to prevent the spread of a harmless virus. Madagascar in particular, being an island nation with just one seaport, is particularly prone to hair-trigger quarantines, as lampshaded in a 4chan fancomic:


 * In Tenchu 3, the player can hear the player character's footsteps at all times, even when they're inaudible to guards your character is close enough to touch. This would't strain plausibility too much if the game was first person, but the player's perspective is a good eight to ten feet behind the player character.
 * In Oblivion, the guards don't just focus on you, if you are under attack from anyone and they do not see you strike first they will actually help you out. (E.G If you got too close to the pirate ship the pirates don't like it...) Likewise, the stealth aspect is quite realistic, items that make loud noises are noticed (I think, just talking from memory right now.) However, if you move an item by kicking it around or pushing it with another object, the owners of the object will ignore you until you actually grab it yourself. I think I'd object if someone walked into my store, knocked over my armor and kicked it around the floor until I couldn't see what he was doing with it. Likewise, I'd probably notice if said armor suddenly disappeared afterwards.
 * Favourite use for the Weak Fireball spell, you can go to a bookstore and splatter the shop's inventory all over the damn place, as long as you don't accidentally graze the shopkeeper they never object.
 * It's also possible, by ridiculous over levelling, to have a Sneak skill high enough that NPCs will fail to notice you standing right in front of them in broad daylight.
 * Judicious use of Telekinesis magic allows you to pick up an object you want to steal, quickly run to a corner (the shopkeeper will usually follow you, but you can get out of sight for a half-second usually) and grab it, they won't notice that the object disappeared into thin air.
 * Incidentally, if you have a bounty on your head (even if this is a bounty for killing every other killable person in the game, they will confront you first, and if you pay off the bounty or go to prison for a few days, they let you go without further ado. If an NPC commits any detected crime (even stealing a crumpled piece of paper), all nearby guards will try to cut them down where they stand.
 * Also, shopkeepers always know when something you try to sell them is stolen. Unless, of course, you sell it to a Thieves' Guild fence, then immediately buy it back, which apparently removes the "aura of having-been-stolen" from the object.
 * That's what fencing is. Reselling stolen goods so that the new owner can say they purchased it legally.
 * Still, random silver ware that looks exactly like the 1000 other pieces of silver ware you found earlier is immediately recognized as stolen. And the "purchased legally" thing isn't the only way to explain why you are walking around with hundreds of pounds of interior and/or armory when hundreds of dungeons and thousands of treasure hunters are around who can sell what they found in some ruin without problems.
 * Guards won't mind if you steal a horse in the middle of nowhere, ride it up to the town gates, dismount, and go in to the city, as long as nobody saw you steal it. Get back on the horse you just got off of, though, and they will act like you just stole it right in front of them.
 * Similarly, if any guard sees you do something illegal: every guard in the world instantly knows about it.
 * Occasionally guards you didn't even know you aggroed will chase you through miles of wilderness through lakes, packs of feral animals and mountains in order to remind you to "Stop right there, criminal scum!"
 * Skyrim has NPCs programmed for eyesight and hearing which usually work pretty well. Unfortunately it also means that if you put a bucket on a (non-hostile) person they no longer have the ability to see you at all.
 * They also appear to have No Peripheral Vision, especially minor NPCs who will sit down and lock their heads straight forward instantly becoming blind to everything that occurs to their left and right.
 * It's also pretty hilarious to stealthily kill a person and watch their friends run around to find the cause for a minute before sitting down next to the corpse of their buddy and complain about being spooked by the wind.
 * Even funnier is when its said by someone you're trying to kill via bow, whereupon they go back to drinking with an arrow sticking straight through their skull.
 * In Skyrim time won't stop when you start a conversation, so things will continue happening realistically in the background. Unfortunately the conversation will cut off the NPC's AI, so they won't react in any way, even if a dragon starts attacking the village behind them.
 * In Left 4 Dead, a group of people all firing machine guns and shotguns only alerts zombies up to approximately 20 feet away, while setting off a car alarm will draw a large crowd of zombies from outside the player-accessible area to swarm the team. Even worse, the Dead Air campaign features two spots where large zombie groups will be attracted by an electric motor running, but not by a the sound of a gas station exploding or a 747 crashing into a runway, in that order.
 * Justifying Edit ahoy! The horde aren't attracted to loud sounds, but rather high-pitched ones. This is why the pipe bomb has a smoke detector alarm taped to it, and why the zombies swarm it before they'll swarm you.
 * Still, the passenger airliner with the high-pitched whine of it's engines coming to a metal-on-pavement scraping crash is pretty high pitched.
 * Perhaps the Survivors had cleared all the zombies in, say, a two-thousand-foot radius, but the plane crash attracted the attention of a lot of zombies that were beyond it. The crash ended before any of those zombies detected the Survivors so they go back to ambling around, but they are now close enough (say, one thousand feet) to hear the electric motor run and close the distance.
 * Far Cry 2 takes enemies distinguishing the player's footsteps from ally's footsteps to ludicrous levels. In that game enemies can detect you even if you're driving an enemy's boat, jeep, or car, from a distance of up to a half-mile, and through tinted glass. Further, both you and your enemies are foreign mercenaries of all ethnic types, with no distinguishing uniforms or badges, dressed in a variety of casual outdoorwear -- how can they tell you're an enemy in the first place?
 * They can't. The entire country is in the midst of a complete breakdown. It's not that they always know it's you, they will shoot anyone who comes close.
 * In Final Fantasy VII, the room in the Shinra building where Cloud, Barret, and Tifa must hide behind golden statues while guards patrol on the other side of them. The guards sure seem diligent with their work, too bad they apparently have about 10 degrees of sight. For bonus points, getting caught summons a slap-on-the-wrist battle with the guards, and then for some reason after the battle a new set of guards comes to take over where the previous ones died and the whole thing can be repeated indefinitely.
 * If Cloud and team are caught too many times, eventually they apparently run out of guards, and you can just walk to the opposite stairwell.
 * Averted in Splinter Cell, the guards are very alert, and from the third one on, it takes ambient noise into account. Nearly everything you do sets off their alarm if they walk into it. From knocked out guards, to shooting out the lights. Even if you take a more subtle approach and turn the lights off, they'll wonder why it happened. After an alarm has been set off, the guards will never go back down to their original state and will be a little bit more alert than usual.
 * In Splinter Cell Double Agent the guards in several levels are actually at war with others. The prison riot is a good example. You can sneak around pretty easily and sometimes the guard will stop shooting at you if another prisioner shows up.
 * In Splinter Cell Conviction, the guards have difficulty noticing Sam in the shadows, but they will spot him if they get close enough. And alerting any single guard will have him call out to all the other guards before pursuing, putting everyone on alert. However, in many cases, you want the guards to know where you are, or at least, where you were, so that they'll assault your Last Known Position (LKP in the game) while you slip, unnoticed, into an ambush position to take them all out.
 * Many MMORPGs, including City of Heroes and World of Warcraft, have this as an effect of the level system- an enemy of an appropriate level for you to fight will notice you and begin trying to kill you at a moderate distance (Specifically, around the time when you enter long-distance attack range.) On the other hand, enemies much higher level than you will spot you coming a mile away, and enemies much lower level than you will pretend they don't see you even if you're standing at arm's length from them.
 * Some fans argue that lower levelled creatures are simply staying out of your way unless you go out of your way to invade their personal space.
 * Which works fine, for the Defias Brotherhood. Maybe not so much for the starving wolves.
 * In City of Heroes, specifically, this is easily justified by word of your heroic or villainous endeavors getting around. By the time you're 'famous' enough for the people you're attacking to not reward you with anything, they know to leave you alone because you can wipe the floor with them in just barely more time than it would take to avoid them completely.
 * Mobs can smell weakness.
 * In the early days some instance mobs were "intelligent" and when aggroed got help instead of attacking the player. This made the instances too hard and it was later patched out.
 * In Final Fantasy XI, mobs hunt by specific methods: sight, sound, magic use, and low HP are the big ones. Mobs that are significantly lower level than you will still aggro you if you stop to heal in their aggro radius, though. In addition, many mobs have their aggro range altered by the day/night cycle or weather effects. Goblins in particular are damn near blind during the day, meaning you can run right past them most of the time and they won't even blink. At night, on the other hand...
 * Averted in Jedi Knight: Jedi Academy, where there is a level that takes place on a desert world with sand worms. The sand worms will react to anything that hits the sand, including blaster shots and thermal detonators. The latter results in a particularly useful strategy for distracting them.
 * In Wasteland, there is one point where you must sneak past a squeaky board to avoid alerting the guards. However, past the board are doors to open - you can blow them open with TNT and noone will hear.
 * No One Lives Forever - Guards are pretty oblivious to dead bodies - or rather, what caused their death, like a crossbow bolt sticking out of their head. They just ask "You all right?" and continue on their merry way after spending some moments looking for the player and getting rid of the body. They got it right in the second part (and sections of the first) as the guards would raise the alarm, but it was still funny to have a guard, upon discovering his colleague dead right next to him for no reason, say "Wake up!" and when realizing that he was dead disposing of the body and saying "I'm not doing the paperwork."
 * In Mercenaries, players can hijack a faction's vehicle and disguise themselves as a member of that faction. In the first game, this was essentially foolproof, no matter what vehicle you were in, as long as you didn't engage in any hostilities, enemy soldiers would not realize that you are an imposter, unless an officer was present, at which point your disguise would be ruined. This led to scenarios where a distinctive-looking mercenary could be driving along in a stolen open-top jeep, but still raise no suspicion from the North Korean troops, but driving in a tank with no means of anyone seeing you from the outside would raise an alert if you can across one officer.
 * The sequel changes this, where disguises are based on a timer; stay in a faction's view for too long, and they'd realize that you're not one of them. This makes for essentially the same problem; if you are careful, you can drive through a base with nobody realizing who you are in (on a dirtbike, even), despite the fact that ingame chatter shows that everybody knows who you are.
 * Crops up in, of all things, Dwarf Fortress, where the elven merchants who visit you every spring will get extremely offended if you try to sell them goods they deem unethical, like wooden items or animal products. This can result in them packing and leaving in a huff, or even declaring war, if you try to sell them wooden trinkets that you just bought from them five seconds ago. They will also be very impressed if you go without cutting down trees for a long period of time, even if there are no trees for miles around.
 * Crazy? The guards in Tenchu must be Psychic! Splashing in a puddle inside a cave - where no guard can possibly get to, but can hear, causes them to immediately shout "NINJA!" and go into !? mode, no matter how many times you splash through the water. Presumably fountains would be bad for their blood pressure.
 * Scarface the World Is Yours. Getting cops nosing around because of gunfire? Hiding behind some dumpsters tends to work. They're pissed and chasing you? Distance, not line of sight matters. No matter how far and straight the road ahead is, pure speed allows one to evade notice. Possibly handwaved with the game mechanic that bribing the cops in general reduces their interest in you.
 * Prototype: The soldiers reactions to Mercer "Hey there's a guy running up a building, beating up monsters with his bare hands and flying by firing blood out of his wrists, ah, doesn't matter he's wearing a military uniform. Wait, there's a guy in a hooded top! KILL IT! KILL IT! KILL IT!KILL IT!KILL IT!"
 * It's worse than even that; running up a wall attracts no attention, but hand-over-hand climbing the same wall will get you caught in no time. That's right - scaling a wall in a manner that might be humanly feasible is a dead giveaway, but scaling it in a way that's physically impossible without superpowers isn't even worth a second glance.

Western Animation

 * Variant on ignoring sounds not made by the protagonist: in an episode of Batman Beyond, villain Shriek has a suit whose abilities include nullifying sound in the surrounding area. When Batman turns on the machines in a factory in order to mask his own movements, Shriek uses his suit to block out the sounds, followed by another adjustment so that he can still hear Batman moving around.

Live Action TV

 * So, Dexter has made a friend. Good for you, Dex. But perhaps you would prefer not to talk about murdering people in a crowded area in broad daylight? Just a thought.
 * Buffy the Vampire Slayer has the entire student body of Sunnydale High to be extremely ignorant to what Buffy, Willow, Xander, Glies and Oz talk about. Though, this also applies to most of the people in Sunnydale about anything supernatural.

Video Games

 * Ground Control 2: You meant to be sneaking past the guards but one of the guys you are controlling (Rho) keeps shooting at people, unless you get in range of their weapons nobody notices their comrades dying.
 * You can also shoot infantry with scout drones without them noticing.
 * Fire Emblem enemies, at least in FE 7,9,10, often seem to be a bit more comfortable with their current spot then they should, considering the situation. A common tactic against these guys is to sneak until you get a few units just Outside Detection Range, then let a heavy defender (Knight/General) enter, lure the guy out (finally) and let him get the crap beaten out of him. Despite the low movement range of the heavy defenders, you will have plenty of time to set this up.
 * In one of the missions in Golden Eye 1997, there is a point from which you can shoot at the head of a guard who is wearing a steel helmet without him seeing you. Because of the steel helmet, the headshot is not fatal and it takes several shots to kill him this way, with his head jerking backwards with each hit. Because he can't see you, he completely ignores this and remains at his post until he dies.
 * Happens a lot in Warcraft 2: melee units will just sit and get shot repeatedly without making any move to attack the unit that's not even THREE FRIGGIN' FEET AWAY!
 * In Halo the Covenant will do almost nothing while you snipe a them from across the map. The Xbox 360 remake of the first game has an Achievement for sniping all the mooks in the beginning of a level without them noticing anything. Apparently, walking around the corpses of their buddies doesn't count as noticing.
 * In Disgaea enemy units won't move towards your characters unless they're in range and they'll never (in my experience) move away. As a result it's possible to kill some enemies with a gun (the weapons class with the longest range) or magic (which can reach the other side of the map at high levels) without any reaction on their part.
 * The Raiders in Fallout 3 can sometimes be incredibly oblivious, it is frequently possible to blow off the head of a person they are standing right next to without them noticing. In addition, you can sneak around a building, knocking around shopping carts and other junk, but they're never notice.
 * Justified by the fact that they are high 99% of the time that they aren't stealing drugs.
 * A particularly odd example, or possibly subversion, comes from Nexus Prince Shaffar and his guards in World of Warcraft. He has a room full of groups of five or so guards who you kill one group at a time as you work your way up to him. The kicker is that he greets you when you enter the room. He clearly knows you're there and has shouted it out in front of everyone, yet his guards stand there waiting to be slaughtered one group at a time. What are they all waiting for?
 * Knowing the Etherials, payment
 * City of Heroes, and presumably other MMORPGs, will sometimes have an NPC you are attacking break and run, generally toward a nearby group of oblivious NPCs. You can continue attacking the runner with ranged attacks, and can defeat the runner so that they fall down right in the middle of the other group, and they pay no attention to the body lying at their feet. NPCs will also ignore things like eye-searing bolts of fire passing through their group to hit targets beyond them.
 * Laura Bow: The Daggar of Amon Ra has a minor but interesting case. In the armory room there is a large tapestry in the far left of the screen. To collect evidence, Laura can use this tapestry to hide behind it to surprise or remain undetected while other NPCs are dropping important clues at particular time intervals. You can use it from listening to a conversation to leaping out and scaring the hell out of another suspicious party, nobody will suspect anything (although one conversation with Wolf and Olympia lampshades this). However, in Act 5 when Laura is chased by the killer, hiding behind the tapestry regardless of how well it worked before (or the fact you blocked off all possible ways or indication the killer could've seen her hide there during the chase) will cause the killer to instantly know she's there and smack her with their spiked club of doom through the tapestry.
 * Both played straight and Averted in Gothic. All enemies have a specific sight range, and will not react to anything outside it -- so you can startle one monster of the group, pull it away, and kill it without others noticing. However, averted with most humanoids (humans, orcs and lizardmen) and some animals (wolves, rats, scavengers, and some others) -- startling one of them also startles everyone in its vicinity (which, in turn, can startle everyone in their vicinity, and so on), leading to the whole group taking off after you.
 * While sneaking into the Gerudo fortress in The Legend of Zelda Ocarina of Time, the guards pay no mind to seeing one of their fellow guards lying on the ground unconscious, with an arrow sticking out of their skull. They also can't be bothered to look up at the ceiling, no matter how much noise Link is making with his hookshot.