I wonder if Boeing EE's are tracing the route of electrons from the ground power receptacle throughout the aircraft to see how current could leak into the ELT circuit. Let us suppose that they find something that allows a stray voltage to wander here or there. Hmmm.
Other than putting all ELT's in the bin, as some Aussies seem to suggest, maybe one could return the ELT to the condition of entirely isolated from all of the aircraft elecrical systems. This would lead to a pre flight test where a CC or maintenance crew accesses the ELT test switch, and either confirms the light or the flight deck crew are listening up on the ELT's freq and confirm via intercom good test or bad.
Yes, it's a crew coordination drill, but you don't need an ELT unless you are intending to fly in the first place. You always fly as a crew. It may also be in the too hard category, in terms of getting access to a test switch from inside the cabin.
Just thinking aloud here ...