Could it be from the outside shower opening? The backside of my outside outside shower door and the shower hose is in my bathroom sink cabinet. When the wind/rain blows from the right direction it could come in around the door or the frame for the door. The solution would be to remove the screws holding the frame of the shower hose door and pull it out slightly, clean off the existing butyl tape, and put new butyl tape around it and reseat it.
Gene & DW Ginny
2002 Sunline T-2363
2008 Toyota 4-runner 4wd 4.7L V-8
Reese Dual Cam straightline - P3 Brake controller