Alertness Blink

An audio-visual trope found primarily in cartoons, the Alertness Blink is a short chirping sound played when a character becomes aware of something. Usually associated with a flash near their head as a visual cue. Commonly comedic in nature. This trope originated in Western Animation before the advent of dialogue as a visual indicator of shock or surprise. It would often be accompanied by a sharp note.

More often than not, associated with the realization of something that leads to Cross-Popping Veins, Luminescent Blush, Blank White Eyes, Color Failure, or the like. Beam of Enlightenment is a similar idea, but used differently. Idea Bulb is also similar.

One common variation (often found in video games) is a ! rather than a random flash of light. There's also the "little crown" variation, most seen in anime. Compare Confused Question Mark.

Examples of Alertness Blink include:

Anime and Manga

Common occurrence in the Kaichou wa Maid-sama anime. Once by a cat!
Seitokai Yakuindomo, omnipresent.
Mitsudomoe uses it occasionally when Yabe-sensei notices some new hilarity ensuing, or in general use.
The second Minami-ke season has the "!" version.
Ookami San to Shichinin no Nakamatachi has an example in the first episode.
Nuku Nuku TV, episode 9, as seen above, right before Sweat Drops and Amusing Injuries ensue.
Toradora! has it in the first episode.
Used several times in the Bleach anime.
Re: Cutey Honey has it too.
A Certain Magical Index 2 has two in a row from Mikoto in the very first episode.
A couple in the opening of the Yozakura Quartet OVA, as well as a few in the first episode proper.
Squid Girl features it in the OP and occasionally in episodes.
Gundam has a franchise-specific version of this representing psychic powers and/or telepathic communication. It's called the "Newtype Flash" by fans.
- Excel Saga features a Newtype Flash in the episode parodying Gundam.
This happens a few times consecutively when Konata realizes the chocolate is oozing out of her cornet.
Kimi ni Todoke uses this every so often as well.
The World God Only Knows, standard use (such as the main character realizing he just swore to find an invisible girl in less than an hour).
Koe de Oshigoto!, almost always followed and/or proceeded by Luminescent Blush due to the Ecchi nature of the series.
Happens all the time in Yotsuba&!.
In A Channel, for example when Run tells the other girls that Tooru is actually an honors student.
Ao No Exorcist from time to time.
Beelzebub makes comedic use of it.

Comic Books

Spider-Man does this all the time when his Spider Sense activates. It's most frequently depicted as jagged bolts around his head, with half of his mask symbolically appearing.

Live Action TV

The late 1970s Spider-Man series starring Nicholas Hammond actually managed to pull off a live-action version of this trope to indicate when Spider-Man's spider sense went off. They'd intercut photo-negative images with the color-corrected images for a few seconds, creating the same effect as a standard Alertness Blink.

Video Games

In The Legend of Zelda the Wind Waker, the old man on Windfall Island who asks you to look for his daughter makes the sound and flashes the symbol every time you get in range for him to ask you again. Can reach Most Annoying Sound levels, since he stands right outside the entrance to the main part of the island and you'll have to tap though his dialogue if he gets close enough to you.
The ! in Metal Gear Solid is a variation.
Professor Layton also uses ! whenever you click on a puzzle location.
The Player Character in Pokémon Mystery Dungeon: Explorers of Time has one when he or she notices he or she's been turned into a pokemon (both blink and chirp).
Sleeping gooballs in World of Goo chirp and get an ! over their heads when they're woken up.
Happens in the Mario & Luigi series with yet another !.
Nippon Ichi games tend to give their character sprites such a chirp.
The Pokémon games use the ! version on opposing trainers when they see the player and engage them in battle.
Ace Attorney uses the musical note, "!", and screen flashing variations.
The shadows in Persona 3 and Persona 4 get a ! above them when they spot the main character's party.
- Many of the characters themselves get these in drama scenes outside of combat.
In Breath of Fire III, you can try to learn some monster's powers; if the skill is learned, you get a ! over the character's head; if you get a ? instead it means the skill can't be learned.
- In the game's World Map, if you get a ! over your head it means you found a random combat area; a ? indicates a secret location.
Stealth Bastard have enemies going ! when fully alterted.
In Pac-Man Championship Edition DX, sleeping ghosts go ! with chirping sound when Pac-Man goes past them to notify they're alerted.
In Thwaite, the ! over a house that's about to be destroyed, with accompanying diminished Scare Chord.

Webcomics

Spinnerette has it here and here.

Western Animation

Mickey Mouse commonly invokes this trope.
Oddly enough, this appears at one point in the non-cartoony Batman the Animated Series. The alertness lines are thin and almost unnoticeable, which may suggest that that episode's animators misinterpreted their appearance on the storyboard.
One of Chuck Jones' Tom and Jerry shorts plays with the "!" form of this, when Jerry runs off the edge of a building. After realizing there's nothing under his feet, he makes a "?" with surprise, then grabs onto it as the dot part falls away. When he falls, the question mark hooks onto a flagpole, saving him in the nick of time. When Tom runs off the same ledge, he makes an "!" in shock and grabs it, then falls past the flagpole. After noticing it, he bends the "!" into a hook shape, much too late for it to be any help.