Place it at the reveal beat, just as the hidden character is noticed, then follow it with the Ba Dum Tss Drum Sting to turn the exposure into a joke. For a villain entrance, follow it instead with the Evil Laugh as the spotter closes in.
About this sound
This is a bright, piercing electronic sting lasting about a second and a half, built around the alert cue that sounds when an enemy notices the player. It feels tense and suddenly alarming, like being caught mid-sneak with nowhere to hide. It comes from the Metal Gear Solid series, where it marks the moment stealth fails and detection begins.