It means he is wondering what you look like without any clothes.
he likes you... but then you already knew that.
Men are such stupid ******** we think the smile say yeah I hear you and yes I need your ****
It means you nearly got kissed.
It's a Spot-The-Axe contest and you are losing (cos HE smiled!)
I'm pretty sure it means that he should not take another bong hit for a while.
You'd have to elaborate on the "weird smile". Was he shy? Surprised? I'm guessing he looked like "I got this girl to look at me so that means I could **** her if I wanted to." It's a technique to catch your attention, make you remember him (which worked), establish dominance by starting and ending the interaction, display confidence, and make you feel like you are lower social value. Usually he would wait a while then come up to you and put you at ease with conversation. Or if it's a recurring situation like school or work, he may wait several days or weeks. If no recurrence possible, it's probably just practice for him hitting on other girls.