check to see if theres an access panel behind the tub faucets, usually in a closet, etc. if not its time to cut one. you should check the back side of your valves and shower arm risor for a leak first, also the waste and overflow. normally if the leak is a drain it shows up below your floor. if its a water line, stuff gets real wet ’cause it leaks 24/7s. thats why i lean to a faucet or wall problem that only leaks a cup or less every shower. remember, if your floor is soft, you should be planning that well needed remodel now, it wont ever get any stronger, just dry.