Slow water leaks in hidden areas can do major damage before you realize it's happening. This photo shows a slow leak through poorly grouted shower tile. During a shower the water was seeping into the cracks of the grout and into the shower threshold and the bathroom subfloor. Once we discovered this using the infrared thermography camera and moisture meter we were able to trace the damage through the subfloor and into the crawlspace. Some of the beams and joists in the crawlspace were already so rotted they had to be replaced. Wood can rot at 20% moisture and support mold growth at 17%. In this case, the leak had been long-term enough to weaken the whole support system under bathroom. A very costly repair resulted when it could have been prevented.
Make sure all shower and tub tiles are fully grouted and the joints and edges are sealed with caulk. Check this on a regular basis to prevent water seepage through to underlying substrates.
Remember, "An ounce of prevention is worth a pound of cure", Benjamin Franklin