When a guy you know keeps looking and watching you?

There is this guy I'm friends with, he always looks over at me and watches me from a distance, every time I catch him he makes it look like he wasn't looking by turning around or turning his head quick! He is so fast at it that it's almost hard to notice. He especially looks if I'm talking to another guy, and he even notices other guys looking at me and just looks at them back or almost watches to see what my behavior is when it happens.
We're friends we talk when our paths cross he always smiles and makes eye contact or even sometimes put his hand on my back and always asks how I am.
I've noticed he never likes or comments on anything I put on facebook, but knows about what I have there.
He will out of the blue text me not sexual stuff but like maybe a flirty comment or tease me about something we did or talked about that day or the day before at 10-11 at night.
Does he seem interested?
When a guy you know keeps looking and watching you?
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