I ran across the same problem 25 years ago when a handyman had retiled a bathroom in a house. Voltage readings were in the 50v range, just enough to give a tingle and heat the lead on the main plumbing stack enough to cause it to reliquify. Problem was solved when we removed a single pole switch and discovered a pin hole on the hot from jamming the switch back into the box. Reinsulated the wire reinstalled the switch, problem solved.
