Usually, mostly I do it if I've noticed that a cute guy was looking at me and I'm too shy to look at him. if that makes sense?
Maybe you may have worn or done something ridiculous once. Or someone told him something about you that may have been a bit funny (inside joke). Or he likes you
id say he thinks your cute
The smile isn't really an indication of much (could mean anything/could be interpreted many ways), however the look up and down is a bonafide "checking you out" red flag.
I always thought a smile to yourself was a gesture of ridiculousness or something not positive, The last time I did it was to a girl that denied me online that I happen to see in person, and before that it was to a girl who was ignoring me because I never called her back, and if you don't have history with the guy he could be thinking anything. I hate to keep throwing out examples but it could mean you look too easy or he thinks he can do better. I wouldn't want a girl to do that
It means plainly that he likes what he sees, at bare minimum. It may even mean he likes you, yes. Something's sure making him happy, be it you or how good you look. This is a good thing.
Sounds like he's checking you out to me...and yes, it sounds like he likes what he sees