Guy's Behavior

What does it usually mean when you catch a guy staring at you and he doesn't look away?

I know this guy whom i have caught staring at me twice.
When i look around i see him staring at me so i look back to confirm if he is. When i do catch him staring at me, he keeps staring back at me as if nothing has happened. I raise my eyebrows asking him "what" then he replies back by raising his eyebrows. then i smile and look away because i feel uneasy but he still continues to stare at me for I think 2-3 more seconds then stops. What does this mean?
What does it usually mean when you catch a guy staring at you and he doesn't look away?
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