Water damage can go run down the wall.
When you have a water leak in your home if it is not taken care of quickly it can run down the walls and breed mold. The most common causes of mold growing on or in your walls are high humidity, condensation and/or water leaks (which are often hidden inside the wall). Condensation forms when water vapor in the air meets cold surfaces and cools to become liquid (common in basements). Leaking pipes near or inside of walls are a common cause of mold. Many times mold can be growing inside the walls without showing any signs on the exterior of the walls.