When you hide, there’s a chance someone will notice you even if they aren’t searching. To determine if you will be noticed in this way, the GM compares your Stealth check with that creature’s Passive Perception score, which equals 10 + the creature’s Wisdom modifier. If the creature has a bonus to the Perception skill, it also gets added to the Passive Perception score.

Passive perception can also be used as a bar to measure a character’s alertness by.