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.
It is a long apartment. Electric meters for basement and first floor are on the outside of the South wall and cables go to separate boxes on West side of a bay window. One box is in the basement and the other above it on the first floor. Water pipe comes in basement floor on East side of the bay. Two wires attached to the galvanized pipe coming from behind the drywall lining the bay window, so I presume one is from each main panel. The metal cover over the water meter is in the ground several feet in front of the house to the South so there is nothing I can see that puts the ground clamp BEFORE the water meter. There is no readable meter after the pipe coming inside where the water cutoff and the two wires are reachable. It is a row of about 10 houses and I presume they were all water metered the same.
The water pipe with the cutoff and the two wires serves the entire 4 story house, and the water heater, and the water based heating system. Moving toward the rear (North) one is first in the living room, then a hall. On each side of the hall is first the kitchen and then the bathroom on the West side, and the closet for the water heater and the house heater on the East side. In back is a bedroom with the sump pump and a door to outside. The kitchen, bathroom, and sump pump drain into a 4 inch pipe that enters the sewer running down the alley behind all the houses.
The house was built in 1901, and I don’t remember seeing a GREEN wire fastened to the galvinized water pipe. Both wires were stiff. At the time I thought that sanding down the pipe and reattaching might be a good idea and perhaps a more modern clamp. So far as I know these are the only ground wires for the electric boxes. The water heater is gas, not electric heat, although it may have some sort of electric based on/off mechanism. Have to look at that. The stray voltage was in the copper pipes coming out of the water heater. I don’t know what you mean by “the lug inside the box.”
A friend with an electrician buddy forwarded this advice: “bury a metal plate in the ground) with a grounding wire attached. Then connect the other end to the water main.” I asked if pounding a piece of rebar into the very small area of earth would serve as well as a metal plate. No answer yet from him. Your thoughts? Thanks for everything.