Those sure can be a goose chase, i usually go the same route, check all the constant, or heavy loads, probe em'out circuit by circuit.
Check for heat tape..or leaks to ground, i had one call where the snow always melted on the walkway, if the detached garage light was on....
All leaks and squeaks aside, if the utilities start singing "California here i come" we'll all be answering calls like yours '66