Not all water leaks are visible or audible.
The most common cause of unexpected, high water bills is the "leaking" toilet, which often can be a "silent leak". The leaking toilet may be the result of a deteriorated flush valve (flapper in the bottom of the supply tank). You can identify this particular problem by placing several drops of food coloring in the supply tank. If you see color in the bowl after 15 minutes, the flush valve is leaky. The toilet will also waste water when the supply tank water level is set too high, resulting in continuous water waste through the overflow tube. The water level should be about ¾ inch below the top of the overflow tube.