Each Boss room (but not Mini-Boss areas) has a Custodian creature that spawns and patrols the room while the Boss is not currently spawned (and will disappear if the Boss does spawn)
Custodians periodically search the Boss room for players and tamed creatures and will reveal any found
Revealed players/tamed creatures will have a Fire Field dropped on them, will be prevented from Hiding again for 10 seconds, and will likely be attacked by the Custodian