Leaky Mixing Valves in the Shower/Tub
As the mixing valve in the shower ages, sometimes the water inside the valve comes out of the valve. Mixing valves of various brands us various types of cartridges and o-rings to keep the water inside the valve. The chemicals in the water such as chlorine and calcium break down the synthetic rubber compounds in the o-ring, aging it and making it brittle. As the valve gets twisted or pulled, the brittle rubber o-ring cracks, allowing water to escape the valve. A new cartridge will normally correct this.
If the leak falls onto the edge of the tub or the spout, water may get back behind the tile and into the wall cavity. This will lead to mold, wood rot and worse. Keep the tub spout well caulked in any case.