Ken,

I've seen this 4 times in my years, and each time it has been a hot water line leaking.

First one I found was only because the seller was at home and had me remove my shoes. As I walked across the floor, I sensed the heat. Sure enough, they call in a leak specialist and he pin pointed it to the area.

Another time I found this to be happening and no one would believe me. 3 days latter heater water was bubbling up around the wax ring of the toilet.

Tell them to call a plumber now.

rick