Why does he look at me when I’m in a conversation with someone else?

I was having a one on one conversation with someone, and I was the one listening. This guy walks by us and positioned himself behind the person I was in conversation with, and stares at me only. He was not included in the conversation and was just staring at me. What’s the reasoning behind his actions?

Why does he look at me when I’m in a conversation with someone else?
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