I love people, so I tend to look at everyone around me when I'm walking in town. Occasionally I make eye contact; when I do, I usually smile. I realize I will have to change this when I move to DC or risk being seen as insane or threatening.
Today I glanced at one of the requisite smokers outside my class building. He smiled. I looked away, and back - he was still smiling. He wasn't an unattractive guy, either (sorry, Jim, I notice these things).
Oh crap, I've got something on my face, I think. Once I get inside I immediately head for the bathroom and check. Right cheek, left cheek - nothing. Teeth - clear. No mascara smudges. Was he just looking ... at me?
I have always done this. I either don't even notice that someone is flirting with me, or I think I've got mustard on my shirt or something in my teeth. If you're going to flirt with me, you have to be really, really obvious - or really really bad. I remember one occasion involving a young man who looked like he'd just stepped out of a Boy Meets World episode, one with really bad sweaters. I mean, this guy was a worse flirt than I am, and finally I leaned over to Jim - this was before we were even dating - and whispered "help me, please."
For someone so observant, I'm awfully oblivious when it comes to people paying attention to me.