With the main disconnet open insure that the panel is dead (You could just open every brake in the panel but killing the main is safest.), lift the hot and neutral of the circuit in question. With everything in the house unplugged or off and all breakers open, connect a circuit tracer, (fox'n hound) to the neutral and it will track to any other neutral that is connected and you can see it in every box that it runs through. You have to go through every junction box affected and open the neutrals till you break the errant connection. (it will also seek anything that is still on the line) Good luck
