Heh, I came in a bit late, but I'd just like to say I don't think staring a little longer than normal is flirting. I mean, you're not even talking to the person. You could be staring thinking of how you want to watch them burn.
Or, like me sometimes, you get bored and zone out for a bit, a when you come back to your senses, you realize you're looking at some hot chick or something... so uh, yea.