When a guy stands or sits really close to a girl in public, would it mean that he is attracted to her?

I sometimes notice guys sitting/standing really close to me in the public (on the bus, in the subway, etc.) when the bus/subway is otherwise vacant. They would also look my way whenever I make simple movements (such as look up or put my hair behind my ears). Do guys do this when they are attracted to a girl? And if so, why wouldn't they just talk or approach the girl?
I am asking this because I sometimes find these guys attractive and would like to smile at them, but don't want to make the environment awkward if I misinterpreted the situation. So I would like to hear your interpretations of this situation first.
When a guy stands or sits really close to a girl in public, would it mean that he is attracted to her?
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