Why does he look me up and down if he's not interested?

I've fancied a co worker for a while but finally gave up as i felt he wasn't romantically interested and is just generally being nothing more than really nice to me.

However, whenever i walk into a room i feel he makes a councious effort not to look up at me (like it should be natural and he's trying too hard not to) and recently I've noticed him looking me up and down when he thinks im not looking.

Is he checking me out? Did i give up too early?

Why does he look me up and down if he's not interested?
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