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It depends, how familiar are you with him? Most of the time, unless he's a salesman then he probably doesn't know he's doing it, but it means he's paying attention to you. One day I noticed a female Co worker doing it with me, copying my habits and mannerisms, even the words I do, and I already suspected she liked me with how often she stares at me. The experts do say women do it far more often then men do.
Mirrors your body language not really. It's more like mirrors your established boundaries. People mirror one another's body language, like fear going through a crowd, in everyday conversation. If you really want to know if he likes you go up and put your hand on his shoulder from behind to say hello. If he freezes, or changes how he normally says hello he's interested at least sexually.