Several good responses, I appreciate all of them. Let me address some
of the points, expand a bit on the original post.
1) The water is "cold" to touch, but bare feet are not good
thermometers. We have two sons whose bedrooms are in the basement, and
they caught the water mid-flood a couple of months ago. The water was
flowing from under the bathroom platform, and the water was cold, not
hot. The water heater was the first thought because of the pressure
release valve, but it's located in another room 25 feet away. Some
water seeps into that room, but looked like it was water spreading from
the point of origin in the bathroom area.
2) The amount of water was based on a half dozen or so dumpings of the
shop vac used to suck up the water; i.e., 3-5 gallons per load x 6
loads = 20-30 gallons of water. This amount has been consistent.
3) No correspondence with weather at all. New Years Eve night was cold
and dry here. Another time it had rained the night before.
4) No correspondence with water usage, other than it's when water is
NOT in use anywhere in the house, very early in the morning. The
plumbers checked for leaks from the shower, and around the toilet
(which I had already done.) The flooding appears to happen very
quickly, within a few minutes. I'm also pretty confident it's not
kid-related. Our kids are teenagers, and the boys, who have best
access to the bathroom, are the ones who clean it up every time. (We
even keep the shop vac downstairs for them. LOL)
5) Have already thought of the flood alarm, but the chlorine tester
might be a good idea.
Thanks for the brainpower guys, I've run out of ideas, other than
overhauling the entire bathroom. I really believe that the sudden
onset of this problem holds the solution. Just wish the pros had been
able to figure it out.