Non expert opinion... but i'd suggest running a flow of salty water down the plug hole and using a sensitive meter check for small currents. (taking precautions to avoid any shock incase it is a major fault!)
With co-operation from any neighbouring houses etc.. shut everything off and then co-ordinate switching them back on again bit by bit until you find the problem.
Should you perhaps get the Power Company involved as it could be a PME failure?
They usually have someone who's expert in solving these problems at hand!
