Specifically, you can see mobs - mobs sitting in dark rooms, without light sources - using scan, but you can't see the same mob without your own light source if you're standing right next to them. It isn't your light reaching to illuminate them, I don't think, since a player one room from someone with a light source doesn't have enough light to see with. This might be gameplay more than bug, but I think that, for characters without infravision or ocular enhancement, mobs sitting in dark rooms without light shouldn't be able to be seen on scan. If you can't see them close up without a light, why does distance help? ^_^

Just a thought.

Yeah, probably a gameplay thing.