I immediatly check my teeth for food, my nose for boogers, and my pants for the zipper. If all of those are in order, they must obviously want me...
That sometimes creeps me out. Last night, while in line at a fast food restaurant, this guy who was sitting down and eating kept staring at me. Even as I walked out, I noticed his head turn to look as I left.
I usually feel the need to initiate some kind of staring contest, or give them a convincing nod, as if to say keep doin' what ya' doin'.