First, check for a possible leak inside the building, apartment, or home. The most common causes of leaks are dripping faucets or toilets. You can get toilet tank leak detector tablets or place a few drops of food coloring in the toilet tank. If any color appears in the bowl, the toilet is leaking. If there are no toilets leaking, the next most likely reason may be an outside leak or lack of efficient water use in the outside irrigation system.