Thanks Charles, Speedy Jim, and Dennis- As far as I know, there are no
pipes in the slab. The pipes appear to run through the walls. The hot
water tank is in the laundry room There is no drywall there, so I can
see the pipes along the ceiling go down to the washer, and branch off
to the laundry room, again high up along the ceiling. It most likely
then turns down the wall in the powder room to the powder room sink. I
suppose there could be a leak there (behind the wall), but absolutely
nothing is wet, or even damp. I guess I can cut a hole in the drywall
behind the powderoom sink to see if the pipes continue down into the
slab from there, but I can't imagine that they would have run the pipes
in the slab, when it is easier to run them between the joists. These
are the only plumbing fixtures on that floor. Maybe it's wishful
thinking, but generally, they did whatever was easiest (which is why
there's no switch on that porch light, I think)

Does any of that change your thoughts at all? Thanks again..