In a previous home, I had a shower/ tub combo leak at the tub overflow. Even when showering, the water would bounce off of my back and run down the wall, some of it going behind the tub - inside the wall. I thought the seal around the drain was leaking, so I filled the tub and cut an access hole to finally pinpoint it.

My tub had a bad rubber gasket around the overflow drain, and a few dollar replacement part from the hardware store solved it. Hopefully you can find it soon. A leak in your house will drive you crazy!