Why would a guy look at you every time he walks past or is in the same room?

If he's talking to someone else in the office he'll face in my direction, if I walk into his office (open office with other people) he'll look at me and when he walks past the window to our office (another open office) he'll turn to look in at me.

I've caught him looking me up and down before.

Why would a guy look at you every time he walks past or is in the same room?
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