Sensor Suspense

Hudson: I got signals. I got readings, in front and behind.
Frost: Where, man? I don't see shit.
Hicks: He's right. There's nothin' back here.
Hudson: Look, I'm telling ya, there's somethin' movin' and it ain't us! Tracker's off scale, man. They're all around us, man. Jesus!

—Aliens

Sometimes, the only way to keep track of objects is through crude sensors or motion-tracking devices. Such devices generally don't do much more than show some blips while making bleeping noises. Yet, this can sometimes amplify the tension of a scene, especially if the actual visual appearance of the objects being tracked is unknown. The closer the enemies get, the more rapid the beeping gets, until it becomes a maddeningly nerve-shredding screech that more often than not causes heroes to snap at just that wrong moment before the monsters burst through and pulls him under.

A more recent usage of this trope is to have lots of blips on the radar display, or to have a really big blip appear, as a precursor to something really bad happening. Alternatively, the blips might suddenly disappear, which is even worse for the heroes.

Naturally, this trope has some overlap with Bombers on the Screen and Enemy-Detecting Radar.

Captain! The radar shows multiple incoming examples!

Anime and Manga

Gundam—Tends to do this by having stuff suddenly appear immediately before they come under attack. The Bridge Bunnies suddenly yelling "Heat source detected!" out of the blue usually means bad things are about to happen.
Hanaukyo Maid Tai La Verite—Episode 10. Multiple intruders are detected, and a giant screen in the security maid's HQ shows their position as they move through the mansion.
Macross: Do You Remember Love—The scene where Hikaru watches the radar blips representing his squadron's missiles approaching the radar blips representing enemies while his heart is pounding loudly.
Macross Frontier—When unmanned probes are first launched to examine the Vajra threat, all that is visible is a 3D display of their progress, followed by their symbols suddenly stopping and changing to say "LOST."

Film

The Abyss. In the opening scene a U.S. Navy submarine is tracking an unknown underwater object by sonar. The sonar blip is projected on a screen, showing the object maneuvering near the sub.
- Sonar does not work that way, naturally. Click here for an example of what a sonar operator is actually looking at, or watch Hunt for Red October... not surprisingly, since it's a movie based on a Tom Clancy movie, they went to great pains to make sure the sonar panel was accurate to real life.
Alien—While tracking the alien through the ventilation system.
- Aliens—Preceding the "They're crawling through the ducts!" scene. There's also the scene where they watch the turret's ammo counter go down.
Close Encounters of the Third Kind has the air traffic control scene, where the aircraft (and possible UFOs) are represented not by blips as such, but by basic text and graphics on radar-like screens.
- This is Truth in Television, as that scene was filmed at Los Angeles ARTCC with actual controllers and radar displays. Modern ATC radar (especially at regional centers even in 1977) uses computer generated data to display targets.
Das Boot inverts this trope to great effect, as the sonar pings bouncing off the boat are being used to showcase how the Allied navy is hunting them.
Colossus: The Forbin Project does this with great tension as the supercomputers launch a nuclear exchange between the US and USSR, all the top heads can do watch the radar and hope the machines will intercept the nukes.
Done in Dr. Strangelove, when the missiles are approaching the nuclear bomber.
Fail Safe—The story is largely told via humongous radar displays in the American war room and the generals' reactions to what they see on it. Aside from a few stock clips of fighter jets, which don't last 60 seconds combined, all of the action is set on the radar screen. Better Than It Sounds; it shares directors with 12 Angry Men.
Forbidden Planet featured this during the third visit from the invisible monster. Radar showed something approaching, but no one can see it...
Galaxy Quest -- Played for Laughs

Guy: "Red thingy moving toward the green thingy... I think we're the green thingy."

This is done on the Americanized Godzilla movie, where the submarine is tracking Godzilla approaching them at a high speed.
Predator II—While the capture team is in the warehouse, the support team outside keeps track of the team and the Predator inside the warehouse using symbols on a screen. The suspense increases when the creature's movements indicate it can see the capture team and is about to attack them.
Reign of Fire uses this to show the parachute squad's fight with the dragon, and it then creatively uses it to show you what happens when you don't deploy your parachute on time.
In The Thing from Another World, a Geiger counter is used to herald the title creature's approach.
Parodied in Tremors 2. The protagonists have rigged up seismic sensors to tell them when the subterranean graboids are approaching. While they're at their base of operations, the sensors start going off as if a huge one is headed right for them...until they look out the window and realize it's instead Burt and his oversized, overarmed military truck.
Used as Book Ends in Top Gun. In both cases the MiG-28s are flying close enough together that the F-14s' radar reads them as half the number of planes that are actually in the formation.
In Independence Day, as a clear Shout-Out to Super Dimension Fortress Macross, the assembled counterattack fighters and ground commanders watch as the little dot representing the President's missile flies all by itself towards the alien ship, wondering if the plan to disable the enemy's force fields worked. It impacts harmlessly against the shield. But then the Prez fires a second missile... everyone watches apprehensively... and it flies past the terminus line of the shield and explodes against the ship itself, showing it's now vulnerable to attack.

Live Action Television

Battlestar Galactica -- "We've got multiple DRADIS contacts!"
Doctor Who:
- "Earthshock": The expedition base camp has a scanner that shows life-signs moving around in the cave system the expedition is exploring. Over the course of the episode we get dots suddenly disappearing (expedition members being killed), a dot that fades in and out (the thing that's killing them, which is alien enough to confuse the scanner), and dots suddenly appearing (the Doctor and friends arriving, just in time to be accused of the murders).
- The dragon hunt in "Dragonfire", a flagrant Alien homage.

Tabletop Games

Warhammer 40,000
- Ciaphas Cain—The fourth book has this, when he's in an escape shuttle that's being attacked by an Orcish fighter (since it most likely has very close ranged weapons its appearance on the pod's radar is the same as it would be for a missile).
- Space Hulk has this as a game mechanic, being heavily inspired by the Alien series. Genestealers initially appear to the Space Marine player only as scanner "blips", each of which can conceal a variable number of the aliens. The number is only revealed when within sight of a Marine.
  - Alien Assault is a computer game adaptation of Space Hulk's rules with an Aliens-esque setting, and also uses the scanner blips.

Theater

Return to the Forbidden Planet—The cast wonder whether they have detected one or two things approaching their crashed spaceship. This leads to the line, "Two beeps, or not two beeps? That is the question."

Video Games

Any modern combat flight simulator when an enemy locks onto your fighter. example Averted in any tank simulator, since tanks mostly have no way of detecting incoming fire.
Alien Swarm—Intentionally, as one of its many Shout-Out s to Aliens
Command & Conquer: Tiberian Sun had the Mobile Radar Array which, when deployed, could track enemies hidden by Fog of War, as well as Stealth and Subterranean units. The suspense part can even come into play with subterranean units, in that you can't tell whether what is about to pop up is a Flamethrower-tank or an APC loaded up with Cyborgs intent on murder.
Command & Conquer: Red Alert 2 had an interesting example in the Psychic Sensor. It had a fixed range, but could detect any unit that had the intent (orders) to go into that range.
DEFCON—The only hint of what might be really happening is the occasional sound of a woman crying.
Defender—The scanner was just a display of colored squares. But seeing humanoid abductions—and the high price of failing to prevent them—brings on the Sensor Suspense bigtime. The Ur Example among video games.
Fatal Frame—Whenever hunting for hidden ghosts, or whenever dealing with ghosts that like to pop up behind you or hide.
Free Space -- "Multiple incoming jump signatures, hostile configuration!"
Half Life 2: Episode 2 in the Final for the silo, the car lights up as Strider attack from every point.
Halo: Combat Evolved uses this in the level "Keyes". As the Master Chief proceeds through a fetid swamp and into an underground Forerunner facility, he occasionally gets unknown motion tracker blips that disappear at the range of the sensor. Finally, after finding Pvt. Jenkin's mission recorder, a whole huge mass of unknown contacts begin to smash at the other side of a nearby door...
- Later in the same level, you meet up with a squad of marines and fight the Flood in the swamp. If you get seperated you may soon find that the last green blip disappears from your sensor.
Homeworld—Playing the game entirely in sensor mode is like this.
Metal Gear Solid series—You will be looking at the radar minimap quite often, and you will most definitely notice when one of the blip's vision cones turns yellow.
Silent Hill—The radio starts playing static as dark things draw near. Silent Hill 4 instead uses this to indicate that the room is haunted.
StarCraft II—The sensor tower, when first introduced in a gameplay demonstration, was used for this effect.
Sword of the Stars allows you to consult the sensor display in the tactical screen to keep track of BVR enemies, but you need upgrades to actually give orders from it. Somehow.
X-COM series allow motion detectors. A good way to avoid becoming Cannon Fodder when facing alien weapons, but since you don't know whether the blip is from alien or civilian and on which floor, dealing with the results can be... interesting.
In Mech Commander 2 certain active enemy units are shown as blips when out of your vision range but within sensor range. Sensors do not differentiate between individual types of units, only showing Vehicles, 'Mechs and Generic. The game does alert you to 'Mechs powering up in a rather melodramatic voice.
Marathon, a 90s FPS from the developers of Halo: Combat Evolved, also had an Aliens-style motion detector. It only detected things that were moving; an enemy that remained still was invisible to it.
In Mass Effect 3, the planet-scanning minigame gains shades of this—you have to scan for special objectives in contested systems, but too many probes and the Reapers will see through your Stealth in Space.
Several Shin Megami Tensei games, such as Shin Megami Tensei Nocturne and Strange Journey, show a color-coded enemy sensor that does nothing but build anticipation: blue means everything's fine, and attacks will be rare. Yellow means there's a threat around. Red stands for "attack imminent." Sometimes the sensor will stay red for a while, especially in areas where one bad call or one bad turn at the RNG during a random battle can mean a game over, turning players into bundles of nerves.

Western Animation

Jonny Quest episode "The Robot Spy" starts with a scene at an Air Force base. Radar operators watch the approach of the title device on a radar screen before calling a Red Alert.

Real Life

One can only imagine what it must be like to be a military sonar operator, or to have been a radar operator during World War Two.