This is just a general question for the girls. A guy on the street (accidentally or on purpose) makes eye contact with you, you break it quick and never look back: clear message of, "ewww, why'd _he_ look at me?"... right?
No, not at all. In fact, unless the guy for some reason strikes me as being physically threatening in some way, that's usually the last thing I'm thinking. To be completely honest, I'm incredibly shy and when people look at me I'm afraid they're judging me so I look away to make myself less visible. But if it's someone elderly or someone who looks like they're having a bad day, I'll smile and the same goes for someone who smiles at me first.
You never know what they could be thinking. They could just be nervous that they made clear eye contact with a stranger. Or you could simply just not be their type. They could be thinking, hey they're attractive, or hey they're not so cute.