Several years ago a new in ground pool, rebar and vinyl liner, was causing a major confusion. The GFI receptacle at the pool would trip, with the circuit breaker for it in the off position. It would also trip when the wire to the breaker was disconnected. Only the ground and neutral were connected !
A house about 500 feet away had an open neutral and the pool was acting as a partial return path for their service.
This was with the older style GFI receptacles before they added the indicator lights.
It was a real head scratcher until it was tracked down.
Another installation was for a hot tub sitting on top of the ground about 8 feet from the UG service. The utilty company xformer in thee back yard had a loose neutral connection and people using the pool would get a tingle when they stepped out of the tub.
In both cases checking for open or poor neutral connections in the area of the pool located the cause.
I have heard that water flowing on some types on plastic will set up a static charge.
