It Was Here, I Swear

Our hero has found the Serial Killer's lair, complete with messages in blood and newspaper clippings of the murders. He needs backup, so he leaves the scene and informs the proper authorities. But when they get there, the room is bare, with no evidence that anyone was here. All the hero can say is, "It was here, I swear." Sometimes the killer has left some item to taunt him with, or a clue to the next killing.

The hero may not be tracking a serial killer, but could have uncovered evidence of a Government Conspiracy or the plan of some Diabolical Mastermind. It may even be evidence of some form of The Masquerade, or a Not-So-Imaginary Friend. The key thing is, the evidence won't be there when he returns. The fact that the witness loses all ability to communicate rationally doesn't help.

If the witness hadn't told the other person anything except "You've got to see this", then — after some moments of wide-eyed bewilderment — he may be reluctant to inform them of what he had really seen, for fear of appearing crazy or dishonest; he'll just say "trick of the light, I guess; sorry". The cleaned-up room may contain some mildly interesting thing that wasn't there before, leading the authority character to say "You rushed me here to see a butterfly?", or "Um, that's not a vampire, it's a picture of one."

Some Men in Black shows have actual divisions called "Cleaners" or "Sweepers" for whom this is their entire job. To show up (in black vans, always) at a location filled with alien gore and debris and completely clean it up and remove all forensic evidence in 15 minutes or less. Maidservice on Steroids. Also, witnesses get Laser-Guided Amnesia or disappeared. Sometimes The Men in Black will ensure the incident will be Mistaken for An Imposter.

Of course, the heroes will never actually walk in on the sinister government mooks or the brilliant serial killer in the process of cleaning everything away and thus catch them even more red-handed because there are No Delays for the Wicked.

Technological advances may eventually make this trope obsolete; after all, who today (in the First World, at least) doesn't have a cell-phone with a digital camera feature? Of course, writers already hate cell phones.

Compare Cassandra Truth, Devil in Plain Sight, Nothing Is Scarier, Not-So-Imaginary Friend. See also The Little Shop That Wasn't There Yesterday. Contrast Crying Wolf. Can involve a character's friend who just got turned into an Unperson.

Anime and Manga

 * In the backstory of One Piece, probably the grandest and most tragic example of them all happened to the explorer Montblanc Norland, where he finds a legendary gold city on the island of Jaya, but when he goes back with the king of his homeland in tow, the island is gone (most of it, at least), having been knocked into the cloud kingdom of Skypeia by the Knock-Up Stream some time ago. Which leads to Norland being executed, and him and his descendants becoming the subject of ridicule for centuries.
 * Kanoko's corpse in episode 4 of Ookamikakushi. Unusually, the person Hiroshi tells about it believes him anyway.
 * A Misaka clone's corpse in episode 11 of To Aru Majutsu no Index disappears by the time the police arrive. The police then berate Touma for "prank calling" them. Touma later finds that the other Misaka clones cleaned up the crime scene while he was busy calling the police.
 * One episode of Devilman TV has a man doing this when he sees a severed head. As is typical, it's not there when he brings someone to look at it.

Film
"Joe: It's a dead end. Polly: That wasn't supposed to be there! Joe: IT'S A DEAD END!!!"
 * In the film In the Line of Fire, Clint Eastwood is a Secret Service Agent, on the trail of someone determined to assassinate the American President. His first encounter with him is when a landlord notices her tenant has a "shrine" of sorts to other assassins. He visits the room, but when he comes back with a search warrant, the pictures have been replaced by a single one of him standing behind JFK, a president he failed to protect, a sign that It's Personal.
 * Used repeatedly -- and relentlessly -- in the I Know What You Did Last Summer franchise.
 * The scene where the murderer cleans a dead body and a hundred living crabs from a car's trunk in five minutes without leaving a trace of their being there has prompted a joke that he could have started a very successful cleaning company if he hadn't gone Ax Crazy.
 * Required Secondary Powers Meets Mundane Utility Meets Cut Lex Luthor a Check.
 * Scary Movie mocked the ability of the killer to get away with this so readily. Finding no blood or body at a murder scene, the protagonists argue over whether it really happened. Meanwhile, ten feet away the killer, still in costume, is mopping up a trail of blood before dragging out a trash bag with a leg sticking out of it.
 * Inverted in the original Gone in Sixty Seconds, where the protagonist is the leader of a group that steals cars, and in the process of stealing a car out of a man's garage, he spots them, but they take off. He gives chase at high speed, and is seen by the police. When he is pulled over, he tells the police the truth, that he was chasing a car thief. The police escort him back to his house, where his car is where it is supposed to be. So in this case he's trying to swear that it wasn't there. It seems that the guy they stole it from was the president of a crooked insurance company known for cheating people on claims, so he "temporarily borrowed" it in order to do some payback on the guy.
 * In the film The Spanish Prisoner, Joe attempts to prove that Jimmy Dell actually existed by leading the authorities to Dell's suite of offices, only to find them abandoned.
 * A bit harder to explain is the exclusive club Jimmy took him to which turns out to actually be a public restaurant.
 * In the movie The Game, Nicolas Van Orton is sick of being toyed with by CRS. He calls the cops into their offices, but there's nothing there.
 * In the James Bond film Moonraker, Bond discovers a lab where they are constructing satellites with deadly chemical agents. When he brings "M" and the police back there, everything is gone.
 * Oh, it's worse than that: the lab is replaced with a huge, opulent office. No explanation is ever given for how this happened.
 * The lab was smaller than the office, so presumably one had been kit-assembled inside the other.
 * Subverted in the Get Smart movie: Smart discovers a secret uranium production facility in a bakery. 23 tells him that all that's actually found is a simple (though remarkably exaggerated) bakery scene--despite the fact that Smart, despite his failings, is an agent who pays attention to detail. This is used to imply that Smart is a double-agent
 * North by Northwest provides a slightly more low-key example in which Cary Grant's character is mistaken for a government agent and interrogated with the aid of lots of carelessly poured bourbon; when he alerts the police and tries to convince them of his story, they return to a room devoid of any evidence of alcohol -- or anything confirming what happened.
 * Used straight in The Number 23.
 * The conspirators in Day Of Wrath have the hero doubting his sanity by committing grisly murders, allowing him to come across the scene of the crime, and then cleaning it all up before he can show anybody.
 * In the movie The Manhattan Project, when Paul and his girlfriend try to enter his homemade nuclear bomb in a New York state science fair, federal agents capture and detain them there. Paul eventually confesses that the bomb is in his girlfriend's car, but it's gone when they all go down to the hotel's parking garage to get it.
 * Used to comedic effect many times in various Abbott and Costello movies. The film with the most abundance of it is Abbott and Costello meet Frankenstein, in which slightly-eccentric Costello is constantly trying to inform straight-man Abbott about the existence/location of Frankenstein's Monster/Dracula/a secret room/Dracula's coffin, only to have it moved before he gets there (often within mere seconds).
 * In Arsenic and Old Lace, the hero Mortimer Brewster, visiting his kindly old aunts' house in Brooklyn, is shocked to discover a dead body in the window seat. He's even more shocked later to discover that it's missing (it was taken to the basement for burial). It turns into a Running Gag when a second, completely different body shows up in the window seat, this time brought by his Ax Crazy brother Jonathan.
 * In the first Ghostbusters, Dana sees a temple and terror dogs in her fridge. When she brings Peter to investigate, he opens the fridge to find... junk food.
 * In the Dragnet movie, Joe Friday and Pep Streebeck infiltrate a P.A.G.A.N ritual with thousands of attendees, a fleet of stolen public vehicles, a giant television screen, and a huge pit with a giant snake inside of it. After they escape by the skin of their teeth, they go back there with their boss... and there's absolutely no trace of anything there.
 * In All Through The Night, Humphrey Bogart's character escapes from the headquarters of a gang of Nazi saboteurs during WW 2. When he attempts to lead the police (who are skeptical of his story, as he's a mobster himself) back to the lair, it's been tidied up and all the swastikas, Hilter portraits, etc., have been removed.
 * In Return of Count Yorga, Yorga sends his vampire brides to kill a family of his main target and then kidnap her. They do the deed and leave a murder scene behind which their maid, who happens to be mute, stumbles upon the next morning as well as the living survivor, the family's youngest son. She calls the police, but before they get there. Yorga's servant, Burda, drops by unnoticed and clears the scene so by the time the police get arrive theres nothing left. She tries to get the boy to tell them but he, unfortunately under Yorga's control, denies her claims.
 * Happens a couple of times to Goldie Hawn's character in Foul Play.
 * Happens a lot in the French Fantomas films with Louis de Funès. The titular criminal mastermind loves to do this to the inspector chasing him. In one of the movies, the inspector is staying at a castle. He wakes up in the middle of the night and sees a body hanging in his room. He runs out screaming. By the time he comes back with a crowd, the body is gone, prompting this trope. He next night, he prepares a camera on the nightstand. Once again, he wakes up to find a hanging body. Being smart, he snaps a picture and then runs out screaming. He does, however, make the stupid mistake of leaving the camera, allowing Fantomas to substitute it for an identical one with a picture of the room with no body.
 * Inverted in Sky Captain and The World of Tomorrow.

Literature

 * Matt from The Power of Five becomes victim to this when he's living in the Town with a Dark Secret. Someone who believes Matt is brutally, brutally murdered, and Matt sees the room where it was done. He goes to get someone, and comes back less than ten minutes later. Everything is perfectly in order. Say what you want about the formula of Anthony Horowitz's writing, that was a freakin' creepy scene.
 * In Lud-in-the-Mist by Hope Mirrlees, the protagonist and his friend discover a secret room in a public building lined with mysterious tapestries and filled with (illegally smuggled) fairy fruit. By the time they return with the authorities, the room is completely empty, much to their frustration. It is implied that this is because the first time they entered the room they accidentally gave the correct password while cursing at the locked door, while the second time they didn't remember what they had said and just broke down the door instead.
 * Used in Killer when the girls tell the police about Ian's death, and his body is gone from the forest when they return.

Live Action TV

 * In Heroes episode One Giant Leap, Eden and Mohinder find Sylar's apartment, which has the message, "Forgive me. I have sinned" written in blood. When they return with the police, everything in the room has gone.
 * Also, in "Ink," Matt ends up finding a stuffed animal, a ransom note from a hostage, a mind-read location of the victim and then the body of the victim found under the stairs. When his partner returns to find Matt beating the hell out of the hostage, Matt tells him to look under stairs and the body is gone. The ransom note and toy are gone too.
 * CSI episode Anonymous focuses on the hunt for a serial killer. Grissom concludes that the killer is Paul Millander, who owns a Halloween supplies company. When Grissom leads a raid on Millander's warehouse, it is bare apart from a stool with an envelope addressed to him. It has a blank piece of paper inside, a sign that Grissom interprets as meaning, "We have nothing".
 * "Mr. Monk Is Up All Night"
 * Also used once as a clue. Monk figured it out because
 * The Invaders.
 * Every. Single. Myth Arc. Episode. In. The X-Files. Oh. My God.
 * Notably in 'Je Souhaite', when Scully finally has solid proof of the supernatural in the form of the corpse of an invisible man. Of course, when she brings in the experts to look at it, it's completely gone.
 * Just a few hours later, Scully herself starts to wonder if it was real, much to Mulder's annoyance.
 * In the Babylon 5 episode "Passing Through Gethsemane", Brother Edward encounters the message "DEATH WALKS AMONG YOU" scrawled in blood on a bulkhead; it's gone when he tries to show it to Garibaldi. In fact, the message was
 * In Seinfeld, George Costanza gets himself invited to a night club populated almost entirely by beautiful model-types by... well, It's a Long Story. Once he shows up after losing his 'credentials,' the next day the same building is devoid of anything but a meat packing plant.
 * The gremlin that William Shatner sees in The Twilight Zone episode "Nightmare at 20,000 Feet".
 * Defied in Psych, when Shawn refuses to leave a corpse at its dump site because it'll be gone when they get back.
 * Played straight in the episode "Extradition II: The Actual Extradition Part". Despereaux breaks out of prison to commit a crime, but by the time Shawn manages to report him to the authorities he's slipped back into his cell. The entire escape apparently went unnoticed.
 * A variant appeared in Pushing Daisies where the body of  undergoes a mysterious vanishing act from its grave, along with accompanying evidence that would have linked Emerson and Ned with his death. The body is later found in other (false) circumstances, planted by   to throw suspicion off Ned and his friends.
 * Inverted in Season 1 of Life: the hero, Det. Charlie Crews has a locked closet in his home where he assembles evidence against the conspiracy that framed him. The DA's office obtains a search warrant for a related murder, and Charlie gets home too late to stop the search, but when the cops break into the closet, all the evidence is gone, having been removed by Charlie's roommate, Ted.
 * There's an earlier episode in Smallville where Lana is chased by the "ghost" of a childhood friend. Said ghost turns out to be a clone, and they find a room filled with lots of clones of the same girl. But when the police gets there, guess it, it's goooone. The sheriff even tells Clark that "David Copperfield must have gotten there first".
 * In one episode of Life On Mars, Tyler tries to prove he's not crazy by showing off many parts of his life that suddenly disappear, including Windy's apartment, where he informs her he wants her to talk with the people, and then when he brings them by to do so, the entire apartment is empty and Windy is nowhere to be found, nowhere near enough time passing by and making Tyler further doubt his sanity.
 * The pilot episode of Stargate Atlantis, Sheppard and Rodney find the shuttlepod bay (eventually named 'Jumpers'), upon finding that Sheppard can pilot them, Rodney runs off to find Weir to inform her of the revelation. When they return, the Jumper is missing. After Rodney gives this tropes name, John uncloaks to impress Weir.
 * Subverted in an episode of Leverage 'Three Days of the Hunter Job'. Nate and the episode bad walk into what she thinks is the apartment belonging to someone who is unraveling a government conspiracy, to find the material gone and Eliot coming out, cleaning up.
 * Doctor Who:
 * Used in its standard version in Black Orchid with the Doctor finding a body and trying to tell someone about it.
 * Inverted in Shada where a man tells a police officer that a room has been stolen and so isn't there and of course when the policeman looks, it is.
 * In the second episode of Sherlock, John discovers a wall painted with graffiti that is vitally important evidence. By the time he finds Sherlock and brings him back, however, the evidence has been wiped clean.
 * One episode of The Rockford Files features a Stalker Shrine for Beth Davenport mysteriously vanishing before the police can see it.

Theatre
"Macbeth: Behold! Look! Lo! ...If I stand here, I saw him!"
 * Older Than Steam: An interesting variant occurs in Shakespeare's Macbeth, when Banquo's ghost appears during Macbeth's big banquet. No one else can see it, of course, and then it disappears while Macbeth is frantically trying to convince his wife that it's there.

Video Games

 * In the first chapter of Higurashi no Naku Koro ni, Keiichi stayed home from school because he was starting to get suspicious of some classmates. Two of the girls from his school showed up that night to bring him some food and tell him they hoped he was feeling better. While eating the food, he choked on a sewing needle baked into a pastry. After having a talk with the police, the half-eaten snack containing the needle was nowhere to be found. Its unexplained disappearance would be creepier if it weren't the sort of thing his parents could've reasonably thrown out with the garbage.
 * The hypodermic syringe is another, slightly creepier case in this arc, although at that point, of course, no one was left to actually say It Was Here, I Swear.
 * This trope is arguably subverted in both cases since.
 * Arcanum's (in)famous X-Files quest ends this (as well as You Have to Believe Me) way: when you try to expose the conspiracy, you realize your proof was just, let's say, stolen. For added trauma, when you return to the secret facility where you found it, there's nothing, not even a brick.
 * One of these events marks the halfway point in Policenauts' plot. For added humiliation, it's revealed
 * In Nancy Drew "The Final Scene", Nancy's friend is kidnapped and she knows the friend is being kept hostage in the building she's staying in, but the police don't believe her. She sees her friend tied up in a hidden room through a peephole, but by the time she gets there her friend is gone. There are still pizza boxes and her friends' shoe in the room and so she calls the police. However, she is later told that the police didn't find any of the things she found.
 * Max Payne comes across an operation in progress, eliminating members of a conspiracy and any evidence of their existence. The assassins are even called "cleaners".

Webcomics
"Ivan: Forget it; if I go back for my camera, this will all be gone when I get back."
 * Referenced - with an unusual level of Genre Savviness - by Ivan Bezdomny in The Wotch, when he finds a cult of militant feminists in a secret sub-basement of the school.

Western Animation
"Homer: Uh, is this going to be like one of those horror movies where we open the door and everything's normal and we think you're crazy, but then there really is a killer robot and the next morning you find me impaled on the weather vane? Is that what this is, Lisa?"
 * Parodied in an episode of The Simpsons, "Grift of the Magi", where Homer displays an unusual level of trope awareness:

"You'll be sorry!"
 * Used in the episode "Hungry, Hungry Homer", where the Springfield Isotopes' owner removes the evidence from his office closet. Just a trombone player giving him an appropriate flat note.
 * Subverted in another episode, along with The Little Shop That Wasn't There Yesterday. When the family asks Homer where he got a cursed monkey paw, he says "I got it from that stall that was right over... there..?", realizing he's pointing to a empty alleyway on the last syllable. Then, the camera pans, showing the stall, and Homer continues "Oh wait, there it is."


 * Used in Avatar: The Last Airbender when attempting to locate the headquarters of the Dai Li.
 * Some viewers wondered how long anyone would expect the underground base of the Evil Chancellor's earthbending Secret Police to be there once the heroes escaped from it.
 * This problem, used as a comedic device, continually plagues Candace from Phineas and Ferb; whatever amazing and bizarre thing Phineas and Ferb are doing that time, there's no sign of it by the time Candace tries to show Mom. She shows flashes of Genre Savvy regarding it, but she still tries anyway.
 * This happening Once an Episode is the premise of Phineas and Ferb. Phineas and Ferb's creations must have Plot Armor in reverse or something. Candace tries extremely hard to show Mom what Phineas and Ferb did, but the creations are always completely destroyed. Candace even "discovered" a nonexistent sensor that was buried in the family driveway and triggered a creation's invisibility.
 * Likewise, Doctor Doofenshmirtz's daughter, Vanessa, attempts to show her mom (and his Ex-Wife), that Doofenshmirtz is an evil genius, but the evidence disappears. Ironically enough, it's shown that Doofenshmirtz's scheme (usually the B-Plot) is often what does away with Phineas and Ferb's thing, and when his scheme was the A-Plot, Phineas and Ferb did away with it.
 * In The Movie Candace is shown to be believing in a mysterious force that protects the boys. She later shows how Genre Savvy she's become as
 * Defied in Justice League Unlimited season two, where half the season is about combating and investigating Cadmus, the shadowy government organization whose mandate is to prepare to take down the Justice League in the event that they overstep their bounds. When The Question is kidnapped and tortured by Cadmus the Huntress, recently kicked out of the League, goes to Superman for help and becomes frantic out of the fear that they will never find their headquarters. Superman, however, is perfectly at ease because the League already knows where Cadmus is. They have held off on actually attacking the base because they have been quietly amassing evidence in preparation for going public about Cadmus's true activities, and when the secret facility is moved after Superman and Huntress break in to rescue the Question the League know when and where they moved. Batman explains it pretty clearly when he points out that they have been monitoring Cadmus for months, so of course they have kept track of its whereabouts. Ironically, it is only Lex Luthor who is kept out of the loop, and when he betrays Waller and attacks the now-abandoned headquarters Batman uses that as evidence that it was not the League, since they would not have attacked an abandoned warehouse.
 * Doom Kitty is prone to falling into this scenario in Ruby Gloom, where she is especially handicapped by only being able to communicate through (sometimes frantic) pantomime. Played with in "Doom With a View", when she tries to communicate to Ruby and her friends that a pair of ghosts are still in the closet in question, but unfortunately, only she can see them.
 * Used and then avoided in the Kim Possible movie So The Drama: Ron is chased across town by a horde of tiny robots until he reaches the hall hosting the Junior Prom. When he opens the door, the robots hide. It looks like there's nothing there and Erik notes how ludicrous the claim is, but Kim chooses to believe Ron anyway.
 * In The Road to El Dorado, exiled ex-High Priest Tzekel-Kan is planning on revealing his city of gold to appease the recently arrived gang led by Cortez. Upon learning of this, Tulio and Miguel devise a plan that would bury the entrace to the city behind rubble, which would have the drawback of preventing them from ever returning as well. Their plan succeeds, and Tzekel-Kan is taken for being a "lying heathen", with nothing to show for his claims.
 * This is essentially Michigan J Frog's PURPOSE in Looney Tunes. Granted he only gets like one cartoon to himself, but the whole plot revolves around him being found, and performing so that ONLY the person who found him ever sees it. Any time the man is actually about to get someone to witness it, he stops singing at just the right moment. Then the man is left to try and insist on his super special singing frog, only to be assumed a loon.
 * A little clarification: This was the premise of BOTH shorts Michigan J. Frog appeared in, "One Froggy Evening" and the sequel, "Another Froggy Evening." In the first short, the box the aforementioned frog was found in contains a message stating that M.J. is part of a particular "species" of frog that sings only for their owners.
 * He also showed up once in Tiny Toons with a slightly creepier variant. He's dead (or so it seems) and scheduled for dissection by Hamton, who is the only one he'll sing for. Whenever somebody else looks, he immediately croaks.
 * This happened in the Adventures of the Gummi Bears episode "Toadie's Wild Ride". Tummi is the only Gummi in Gummi Glen to have seen Toadie enter the glen, but because he had been lying about who ate the cake that Grammi made earlier, the other Gummis initially don't believe him about there being an ogre in the glen. Subverted at the end when the rest of the Gummis finally see Toadie when he tries to make off with their supply of Gummiberry juice.
 * Taz-Mania: Taz's attempts to convince Bushwacker Bob that someone is trying to murder them in "A Midsummer Night's Scream".
 * Repeatedly subverted in Adventure Time episode "In Your Footsteps". Each time the bear does something strange, someone acts like this trope is in effect, only for it to turn out everyone already believes them.