When walking alongside a wall, the fog of war clears beyond the wall as well, sometimes revealing enemies of which you should be unaware.
SAVED GAME: link.
Load the attached saved game.
Observe that the door at your left is closed and the room is covered by FOW (it is currently unentered/unexplored.)
Further observe, however, that an enemy is visible beyond the wall owing to the party having walked alongside it.
(Yeah, this one is as simple as loading the saved game and observing; no need to do anything. Sometimes life is easy )