I smile at strangers to be polite and friendly, it can be nothing more than that. Although workplace romances can compliment things so maybe look someplace else.
There is a girl at work I like, I'm not sure if she likes me or not. She usually smiles at me and sometimes waves. Do you normally smile at a guy if you are interested in her? Or is that just being polite?
Nothing wrong with being raised with manners. If you're not sure if she likes you, go up to her and say hi - want to go get a Coke sometime? Never be afraid of finding out; it really limits you. Even if you're shy, just take a deep breath and just talk to her. If nothing comes of this, you'll have found someone you can say hi to everyday at work and you'll have taken a chance, which is something most people are afraid to do.