It turns out the leak in the bathroom was from the shower behind the wall.
But thankfully it was right on the inside of the knob. This means the plumber was able to remove the knob and face plate, tinker around in there and put everything back where it was. Although, we did need a new part or two -- which, of course, didn't come free. But think of all the money we saved by not having to put in a new wall!
So, the walls are intact and there's no drip-drip-drip anymore. Hopefully everything will remain that way.