What does it mean if a guy smiles at a girl?

There's this guy I likes in my school I liked him we didn't know each other , I used to look at him quite a bit there was only like 6 weeks left of school and he'd look back. Then after a while sometimes he would smile at me like we were both eating a sandwich but were in our groups and we were both looking at each other he was smiling a bit. His friends used to look at me a lot when they see me look at him when he wasn't looking. Then when he was finished school for good he came back to clear out his locker he saw me and smiled. I think he did like me He did smile at me other times but he didn't always smile. He "accidentally" bumped into me a few time too

But now he's left I'll never see him again. I added him on facebook though gonna try and talk to him there.