Are the Flanges (escutcheon plates) sealed to the wall where the Shower /Bath taps (Fossets) penetrate the wall? are the Body washers in good order? are the “o” rings not leaking.
Test the system by capping off the bath & shower outlets ( you will have to remove the bath spout & shr arm) & fit a ezy cap or a screw on cap, then turn the cold or hot water tap on. you will then have to visually inspect the penetrations (where the taps come out of the wall) to find that they are not leaking. Also check the weap holes.
If these are ok, then check the wall & shower screens are not leaking.
You may have to install a snap vent on the opposite side of the Shr enclosure to find the leak.