Ok, good information. The stuff behind the tile is definitely not like sheetrock, it is not smooth. It is hard and could be the good cemented stuff. A guy at John B's suggested the possibility of a small leak as water is coming in, something about blow-by such that a small amount of water seeps out and back. Will have plumber come back to check out this possibility. I am still worried about the tub:showerhead wall joint. My husband forgot to clip the curtains last night, and it just really flows down into the basement. It is not as bad if you clip the curtains. I suspect that what happens is that some can still get back behind the curtains and that is why you still get some leakage.
I know the tub spout pipe was a problem at one time. When everything was put back together after the tiling job, then I had to buy a new piece of pipe, like a little extension pipe to screw the tub spout in to. The reason was, well it is hard to explain, it was like the pipes come in 2.5 inch or 3 inch long, one was a bit too short and the other was a bit too long. The one that had been in there for 30 years was 2.5 inches, so it was a bit too short, and yes this I think had been part of the original leaking problem. Another thing was that the new tiling may have been a bit thicker so that whereas the 2.5 inch connector pipe had been ok for 20 years, it was really too short with the new tiling. So I bought the 3 inch long connector thing, and they could screw the tub spout all the way down (pipe tape and all sorts of goo in there). the only thing was, the back of the spout was now no longer flush with the newly tiled wall because of the longer connector. So, what happened was that the tiling guy pulled back from the back to pull the faucets etc a bit further back, and put in a block of wood to keep it back, then sealed up the whole thing.
THe only thing is that this whole thing has now been replaced, new tub spout, I suppose the block of wood must have fallen off when they put in the new plumbing, and I doubt that 3 inch connector I bought is still there. Anyway, I am bringing this up here because you are a plumber. Hoping to get tiling advice from the other forum, but it is hard to understand those guys. Mastic and rock and CBwhatever board. Ciao for now and enjoy your Thanksgiving. Ruth