If an attractive girl was around a lot, for whatever reason you saw her a lot but never talked to her and she always has a serious look on her face and doesn't really pay attention to you/anyone else... would that drive you crazy or would you just forget about her and assume she's mean?
I always have a serious look on my face when I'm not talking to anyone else. I guess I'm just thinking. I don't want to be smiling at myself, it just looks strange. When I make eye contact with passer-bys, I don't really smile, including at my crush. If I see him, I just look into his eyes until he passes by, to see if he looks back.
Those kind of vibes come off as her trying so say "I'm not interested". It doesn't drive me crazy though. If she's not interested that's fine there are plenty more girls out there.
Don't read her on the surface she may have a lot going on in her life else where. People assume because someone doesn't smile something's wrong and there very well could be, but not between you. If she's not very happy in life elsewhere such as at home and it's always things never change it may be she's not a smiler because she isn't use to doing it when she hasn't had a reason to. If you're a fairly outgoing guy pay her a compliment and take her reaction as your feedback. If you're a shy guy just be nice to her in every way you can. She'll probably sooner or later put things together hopefully and know that you like her. If not you haven't lost her if you never had her to begin with. And had I only taken my own advice you see here when I was younger I'd probably have a girl too :D