A unit can sense another unit nearby without seeing it, if a unit triggers it, or just because its within the acquisition range set for units specifically in the map editor. I've had a unit run across a map because I attacked another unit, no way in hell it coulda seen me.
Also, if you're dealing with NPC Hostiles, make sure you put on npc camp, or else, they will run at buildings etc from not far, and they will wake up if you come near/build/do triggering things. :rollie