The hottest spot seems to be in the area close to where the powder room sink drain goes into the floor. The drain pipe itself is not hot, just ever so slightly warm from conducting the heat up from the floor. It's barely noticeable, actually. The floor, however, is VERY warm to the touch- I would estimate somewhere between 110-120 degrees. I felt the heat even through a bathroom rug (and the tile)! I put a thermometer on the floor at what felt to be the warmest spot, and the air temperature at the floor was about 15 degrees higher than the rest of the house (86 vs 71).
The further you get from this area, the less warm the floor. I do not see any pipes (other than the drain) going to the floor. There is a drain in the laundry room for the washing machine. I do not know what path the drain takes once it is under the slab. The main drain stack is on the opposite wall. The main stack pipe did not feel warm.
I thought I heard the sound of water running in a pipe. This sound was coming from the wall nearest to the warm spot (wall is shared with my next-door neighbor). I shut off everything in the house that made noise, and put my ear to the wall. The sound was defintely there, sort of a hiss. I next looked at my water meter, and saw that the tiny white spinner on top was turning. There was no water running in the house that I know of.
I checked with the neighbor, and there is nothing odd going on there.
We have heat pump heat, and the coil is located on the wall opposite from the hot spot. No smoke or odd smells.
I am stumped, but I have one possible theory, which is that I must have a small section of radiant heat under the powder room that I have no control over. This wouldn't totally surprise me, as for example, I have no control over my porch light. The porch light is on a light sensor, and there is no switch- just the breaker. So, anything is possible in this place. They did some really odd things.
Any ideas of what is going on here? Thanks.