All too often the overflow on the bathtub is loose or worse yet loose and the gasket damaged. If the bath is filled up to the overflow and it is loose the water will go down behind the tub which if it’s on the second floor could damage the ceiling. The water can also create a mold issue hidden from sight but nonetheless an issue that can affect the health of occupants. If it’s just loose, a screwdriver will remedy the issue in a matter of a few turns. If the gasket is damaged this is again a simple replacement piece that should be available from most hardware stores.