Sighting Question

[Chipmunk]

Can anyone help me with a little someting I don't quite understand about the sighting rules? I'm often attacked by units I can't see, I don't mean units that haven't been spotted I mean by units that are in hexes my target unit can't even draw a line of sight to (ie if I right click in the direction of the firer he is occupying a dark hex). In a game I played last night I took fire from an initially unspotted enemy tank. After he fired a few shots he somehow appeared on the map even though he was in a dark hex. In my turn his hex was still dark to the unit he had fired on and I couldn't target him with it although he had had no problem targetting me. Both units were tanks of the same size and had the same level of sights. Not sure I understand whats going on here.

Thanks in advance.