In that exact situation, (speaking from when I was single) I'll take a quick glance and then keep walking. It's creepy to do much more than that. Now, if it was a public situation but one in which we were both near each other and stationary I'd take a couple stabs at establishing eye contact. I'm tall, muscular and a decent looking dude so even though these girls have trained themselves to not to humor or encourage contact in public situations, I still get mutual eye contact a decent amount of the time. If she makes good eye contact I'll think of something to go start a conversation about and take it from there. That is if I'm in the mood of course. Sometimes I just wanna get my groceries (or whatever) and go on about my way and don't want to bother with women.