This whistle is widely recognized from Kill Bill: Volume 1, where it is used during a memorable hospital scene with Elle Driver. Online, the tune spread far beyond the film because its bright whistling clashes so well with tense visuals. Creators use it to signal danger, sneaking, revenge or a character acting suspiciously. It remains popular because audiences instantly understand the mood, much like the Metal Gear Solid alert is used as quick shorthand for being spotted.