When a guy smirks as he walks past...what does it mean?

Say when he was just passing you, almost like a small laugh but there is nothing funny to laugh about. I had my back to him and only when he went past and did this did I look up and realize it was him.

He has done this a few times now and I can't help but feel he is coming across as a bit cocky, does he know I like him and that's why he finds it funny? History: we have had eye contact for a while now, but lately I figured he is waay out of my league so I have stopped the eye contact (well, with the exception of a few quick glances). Any thoughts would be appreciated!
When a guy smirks as he walks past...what does it mean?
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