Usually leaks in walls make for wet walls. Is the wall wet? I suspect your leak is underground below the slab.
There is a tool that Licensed Plumbers have to help them detect underground water leaks. It is called an electronic pipe locater and leak detector. By listening to the water service pipes that supply each fixture a qualified person can determine which fixture the leak is near. Once determined the pipe is located and then the leak is located.
The Local Plumber
Tustin, California http://www.TheLocalPlumber.com