I found a small puddle of water on my bathroom floor where there shouldn’t have been any. Chasing the moisture turned into this.
The tub faucet has apparently been leaking, dripping past a terrible past tile installation(plywood instead of backerboard with no sealant at the edge), and running down into the wall.
That horizontal there is a sill beam. I won’t even know how bad it is until I get the tub out, which involves ripping out part of two other walls to free it.
SURPRISE! Major bathroom renovation with structural repairs. Oh, you were already renovating another room, and replacing badly rotted windows before the winter? Tough luck!