The leak is obviously inside the wall between the control valve and shower head or it would leak all the time. There is usually a piece of copper pipe running from the valve to shower head. There is a possibility the leak is in the solder connection at the valve. Depending on who installed the valve and its type you may be able to see the leak by removing the piece surrounding handle. If it's there you resolder it. If you can see water dripping from above the opening for the valve you're back to opening the wall. Good Luck
