To give an example, the family had a water leak in a 50 year old shower, had been used for decades by my grandmother without problems until she passed away. When another family member used it they redirected the shower head to the valves/handles on the side to wash their hair and water went behind the flooded the ceiling below. Don't know that until you see the leak in progress.
Yup, which is why I didn't say that's the problem, gotta open it up to find the problem.
I'd say most all the water goes down the drain, hardly any is going through the grout, there's some and it will cause mold, but he's seeing a lot and soaking the drywall. Could be a bad drain connection, who knows.