Plumbing, always fun, in the home or trailer.
I have a shut-off valve where the water flows in to the waterheater, and no valve where it flows out. This should normally take care of things.
I have had single-control fixtures in my house that leaked not from the spigot, but from one side to the other, so that some hot would leak to the cold water side, and I suppose vise-versa. That could be your case.
I also had a house with a recirculating pump for the hotwater side. All the hotwater fixtures were linked in a loop, and the return was connected to the coldwater input to the hotwater heater. I believe this also allowed hot water to enter the cold water lines feeding the house. When my waterheater was replaced I had the guy install a check valve so this couldn't happen.
2005 25' Safari SS
2005 Dodge Ram 2500 Turbodiesel
WBCCI #3580 - Region 12 NorCal