I just returned from a business trip and have a number of questions, but too tired to ask them. One, however, I am not clear on your suggestion to ground the faucet from underneath the sink. The faucet fills the bathtub, comes out of a tiled wall and the tub is set in a tiled concrete basement floor. Of course a refrigerator is an electrical appliance; a bathtub is not. More after I have a chance to go over all this with my electrically inclined friend. At any rate, thanks for suggestions, both of you.
I was thinking faucet on a sink, not over a tub.
Look very closely at the main ground connections related to the disconnect panels. My guess is that it is related to the other panel because you noted that the stray voltage was there when one panel was off.
Is the main ground clamp on the pipe that goes to the tub faucet? Works best, and belongs on the main water supply to the building, coming from the city or the well.
Should be one for each panel.
Sometimes goes to the electric meter, code varies.
The ground clamp belongs BEFORE the water meter on the city side
Should be an obvious heavy wire (GREEN?) going to a clamp.
Power down everything, remove the clamp, sand the pipe and clamp until it shines, replace and tighten clamp and wire connectors.
You mentioned hot water heat. The ground should NEVER EVER connect to the heating system. They can stand alone and not have a true earth ground.
Most water heaters use a dielectric union, when they work, may not allow stray voltage to pass.
Also the lug inside the box.
Some use both potable water pipe ground and grounding rod, if you had that setup and the ground rod was changed (or clamp loose) and if, if... could be the problem.
Let us know what you find.
YMMV.