Why do some places have a "feeling" the moment you walk in, before you've noticed anything specific about them?

You walk into a room, a building, sometimes a whole town, and something in you reacts before your conscious brain has registered a single detail. Sometimes good, sometimes deeply off. What's actually happening in the brain in that gap between arriving and consciously noticing anything? Is it real information the body is picking up, or is it something we're projecting?