Guy's Behavior

WHY WOULD A GUY STQRE AT MY FACE AND WATCH ME IN FRONT OF HIS GIRLFRIEND?

So I started a new job, and there is this guy at mh workplace with who I've talked on some occasions just asking questions related to work and he'd explain. I've noticed that he stares at me many times and watches me specially when i'm interacting with other people and specially guys. There was a time when i was speaking to a guy who was somewhat flirting and he just stopped the work he was doing on his laptop and completeley focused on him and stared at him while he was talking to me, with a very intense face. So imagine my surprise when i find out he has a girlfriend who just wasn't coming that short period, and even after she was there he would be sitting with her smiling and holding hands and at the same time watch me and watch what I'm doing from accross the room with her right there. But what creeped me out is one time I got into the elevator, upset that day if it's relevant, and they were there, him and his girlfriend, and he kept staring at me the whole ride and observing my face and my body language with his girlfriend right beside him while i prentended not to be aware. Why would he do these behaviours and what does he want?
WHY WOULD A GUY STQRE AT MY FACE AND WATCH ME IN FRONT OF HIS GIRLFRIEND?
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