Why would a guy stare at you with a smile when he thinks you're not looking at him?

Why would a guy look at you with a smile when he thinks you're not looking at him? I mean, when you send a smile to a person, that is a non-verbal sign - "I like you" or "I'm being polite" or whatever other message depending on the context. But why smile when you know nobody is looking?

Why would a guy stare at you with a smile when he thinks you're not looking at him?
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