I take it you didn't know this person. It could be habit or they're shy or both. I don't like people to think I'm staring at them and tend to do this and I usually tend to be very shy around people I don't know. I don't mean anything insulting by doing that. Now if someone flips "the bird" or gives you a sour looking face then I might be a little concerned if I was you. As it is accept the smile:)