Imminent Danger Clue

"Lochley: "Some alien weapons use a hell of a lot of energy to build up right before they fire. Smells like ozone."

Gideon: "You don't have a gun on you, do you? I had to check mine at customs."

Lochley: "No. I never needed one down here until you came along."

Gideon: "What makes you think this has anything to do with--"

Both: "--ozone."

(Both duck as an alien weapon fires at them)"

- Babylon 5 Crusade

To an unaware observer, everything appears perfectly normal. Either no one's around, or everyone's going about their business as expected. There's no reason to think there's any danger nearby or that anything is out of the ordinary.

But there's one tiny little detail that suggests otherwise. Something out of place. Something... amiss. It could be a moved piece of furniture, an insistent background noise, or a bit of speech of behaviour that is vaguely wrong. Whatever it is, it's so innocuous that our hero doesn't think much of it at first.

But at some point, the character has a sudden realization: that detail is a sign of imminent danger. And sure enough, that danger will manifest itself within seconds. The character will either be captured or killed, or will survive through fighting, escaping, or ducking that giant log right before it smashes into the wall.

To qualify for this trope, the detail must be unremarkable outside of story context, and must signal an imminent threat. Due to the Theory of Narrative Causality, that threat will not materialize until the character has made the link -- even if the hidden ninja assassin could have sliced his head off as soon as he walked into the room.

Sometimes preceded by It's Quiet... Too Quiet. If the danger in question is a bomb, expect a Pre-Explosion Buildup. Compare A Glitch in The Matrix, Something They Would Never Say, That's No Moon, Bad Vibrations, Drool Hello. Contrast Cat Scare.

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 * The Laser Sight and Crosshair Aware tropes are almost invariably used to provide characters with an Imminent Danger Clue, even when there is no in-universe rationale for their use, so much so that listing each example here would be redundant. Its the reason that the majority of fictional snipers use laser sights for example, despite how counterproductive it is. Otherwise the only way to give the characters warning enough to survive a sniper attack would be a missed first shot, which undermines the threat somewhat.

Film

 * In The Godfather Part II, Michael anticipates his assassination attempt seconds before it happens because the bedroom curtains have been left open.
 * In Total Recall, Quaid realizes that the Recall doctor's explanation of the trouble he's in is a lie (and that they're about to kill him) from the fact that the doctor is sweating.
 * In Return of the Jedi, Wicket senses an unusual scent in the air seconds before a stormtrooper begins shooting at him and Leia.
 * In Star Trek II the Wrath of Khan, Chekov finds a buckle that says "Botany Bay" and instantly realizes they're on Khan's ship seconds before they're captured.
 * In Die Hard With a Vengeance, McClane (Bruce Willis (Creator)) gets on an elevator with some fake cops. Because he was earlier in the film with some real NYPD cops who mentioned they always play their badge number in the lottery, he realizes at the last second that the fake cop next to him as the same badge number as the real cop. This is enough for McClane to kill them before they kill him.
 * In The Debt, Rachel realizes that the ostensibly unconscious, Bound and Gagged villain in the next room may be escaping when the sound of raindrops hitting pots goes from a three-beat pattern to two (meaning the pot next the the villain's leg has been moved). She goes to check it out, sees the flipped-over pot, starts to creep closer, and promptly gets attacked by the villain, who was hiding just outside the doorway.

Literature
""It was only at the very last, as their fingers brushed across the gap, that something niggled at him. . . was holding out his left hand, why. . .""
 * In the Discworld novels, hoses and ropes getting tangled for no reason are a sign of Auditors approaching.
 * A Song of Ice and Fire: In A Clash of Kings, a character narrowly avoids being murdered by an "ally" on the battlefield in a subversion of Take My Hand:

Live Action TV

 * In Babylon 5 Crusade, Gideon and Lochley identify that an alien is about to shoot because the alien weapon produces a smell of ozone when it is being charged.
 * In Doctor Who, "The Time of Angels." The ancient ruins they're in were from a culture where the population had two heads, but nobody notices for a while that all the "statues" have only one head each.
 * In "The Girl In The Fireplace", the Doctor realizes something is amiss in a little girl's room when he realizes that the ticking sound he hears cannot originate from the room's one clock, which is broken.
 * In the Torchwood serial "Children of Earth," a character realizes she and her son are in danger when a neighbourhood dog stops barking.
 * Sherlock: "Why didn't he close the window when he left-- ...oh. Stupid, stupid. Obviously. He's still here..."
 * Breaking Bad: A story arc in the first season involves Walter keeping Crazy Eyes, a violent drug dealer, locked up in a basement for days while he works up the nerve to kill him.

Video Games

 * In the 2010 reboot of Medal of Honor, the first mission features a bomb rigged to a cell phone, which the men of AFO Neptune barely avoid at the last moment when they hear the phone ring.
 * Later in the same game, a Ranger squad is preparing to secure a small hut bordering on a potential landing zone for their helicopters. They don't quite make the connection, but they know something's wrong when they hear a phone right just before they are about to kick in the door.