[topspin]

I think it's a threat response. When you enter a different area your mind spikes for a moment as you (un)consciously scan around, and whatever thought you were tenuously holding is in jeopardy of getting pushed out. Watch a cat or dog, particularly those that spend a lot of time outdoors and don't take the world for granted; when a door is opened for them they'll step in, stop dead, look around, sniff the air, listen, and then get on with their business. Evolutionary wiring.