OK, this one has been driving me nuts for a while now.
I have a shower/tub on the second floor that leaks down on to the first floor. It only happens with showers, not baths. It takes about 5-10 minutes from the start of the shower for the leak to appear, but when it does, it is pretty steady and yields about 1 to 1.5 cups of water.
There is a handy access to the tub's plumbing in an adjacent room, so I can clearly inspect in the inflow and the drain. No leaks there, on the pipes, wall, or anywhere else I can see. Bone dry.
The bathroom floor is dry after a shower.
The grout around the shower and tub is in good condition, as is the seal where the tub meets the floor.
I have removed part of the ceiling under the bathroom on the first floor. The leak comes from a spot in the floor a few inches away from the corner of the tub. There are no pipes there whatsoever-- just the base floor and the layers of bathroom tiles about it (again, these tiles are dry on top.)
My best guess is somehow water is getting in through the tiles in the shower, and the lip of the tub behind the tiles is gathering the water behind the tiles and dumping it on this one spot. Just a hunch, though. Again, there are no obvious places for water to penetrate, nor any evidence of water moving behind the tiles when I inspect from the other side. But I can't think of any other way for that much water to appear between the tiles and the floorboards. Any ideas?