What does it mean when a man stares at me intensely when I look at him ?

The purpose of my question is to understand a man's behaviour. I was in the elevator and it happened that this man came in. I looked at him and I noticed he was staring intensly for few seconds with his mouth open. Only after looking away , he decided to stop staring. He is a coworker that I didn't get to know yet.

What does it mean when a man stares at me intensely when I look at him ?
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