This is s strange issue indeed! If I had that problem, here is what I would do:
First I would make sure the ignition switch is off, house batteries off, and AC power unplugged. Second I would use a voltmeter to check all the inputs to the circuit board. One should read about +12v. The others should read zero. If I found one reading other than zero, I would unplug it and check the unplugged wire. If it still reads higher than zero, the problem is "up front", not in the circuit board. In all other scenarios, the problem is the circuit board.