A musty bathroom smell usually indicates hidden moisture and mold growth. Check under the sink around the toilet base and behind walls for water leaks. Ensure the exhaust fan works properly and run it for 20 minutes after showers. Look for mold on grout caulk and ceiling surfaces. Replace damaged caulk around the tub and shower. A dry P-trap in seldom-used drains allows sewer gases to enter. Clean mold with a bleach solution and improve ventilation.