From the Core Rulebook:
If you are unnoticed by a creature, that creature has no idea you are present at all. When you’re unnoticed, you’re also undetected by the creature. This condition matters for abilities that can be used only against targets totally unaware of your presence.
Am I unnoticed if an enemy genuinely and correctly believes that I am lurking about, but the reasons for which they believe it are incorrect?
In the scenario, a castle guard on patrol enters a room for the first time that day, finding the door broken open and the room itself a mess. Upon seeing this, they believe some intruder must be nearby. I am an intruder and do happen to be close by avoiding notice, but the room is in its state because another guard, drunk on duty, came through a short time earlier and wrecked the place.
My GM says the guard is aware of my presence, thus making me noticed. I think that would only be the case had it been me who forced my way in and made the mess.