Hiding and Waiting Doesn't Require a Check
Waiting for someone to enter an ambush doesn't require a check from the monster, they would only roll a Stealth check when one of two things happen:
- The party gets close enough that the monster decides to interact with them directly, like by attacking them. This would be Initiative for an Encounter and the monster would be rolling Stealth because they were already Unnoticed.
- If their Initiative Stealth check meets or beats the party's Perception DCs and the party's Initiative rolls (which may also be Stealth checks if some of them were Avoiding Notice during Exploration), then the monster begins the encounter Unnoticed by the party.
- If the monster's Initiative meets or beats the party's Perception DCs but some of the party beat the monster's Initiative check, then the monster is Undetected by those specific party members.
- If the monster's Initiative is less than the party's Perception DCs, then the party knows exactly where the monster is and everything works as normal.
- Unnoticed and Undetected are relative conditions, so the monster may end up Unnoticed by most of the party but only Undetected by the party Rogue with higher Perception. If the Rogue gets a turn before the creature, they may want to Seek for the creature so they can figure out exactly where it is.
- The monster decides to move to a different location while staying Unnoticed, using the Avoid Notice exploration activity. This doesn't require entering an Encounter, though failing the Stealth check would likely be the start of an encounter using this roll as the monster's Initiative check.
The party may also decide to search the area that the monster is hiding in by Seeking the area before entering it if they suspect there might be something lurking about. This might be at a substantial penalty or impossible depending on distance or other factors, as described in the full action's text.
The GM attempts a single secret Perception check for you and compares the result to the Stealth DCs of any undetected or hidden creatures in the area or the DC to detect each object in the area (as determined by the GM or by someone Concealing the Object). A creature you detect might remain hidden, rather than becoming observed, if you're using an imprecise sense or if an effect (such as invisibility) prevents the subject from being observed.
The Search exploration activity covers looking for hidden objects and hazards like traps, though this doesn't change anything for creatures that are hiding. All characters are assumed to be keeping an eye out for hidden creatures, and this is reflected in how the default Initiative roll is a Perception check when something is about to happen that either side might want to interrupt.
