So presumably it was metallic drain pipe coupled together throughout the building with no bonding at any point.
Those multi-dwelling renovations can be quite a problem in many respects. Even with the terraces common in older parts of British towns (row houses) you can find weird things like a socket in one house which has been spurred from a circuit belonging to the adjacent house.
Good if you're the one getting free electricity and using that socket for your heater I suppose, but not so great if you're paying for somebody else's usage!
