I dealt with a slab leak at my Mothers house last summer. As stated before, turn all water off and go look at meter to see if it is spinning. At her house there was water coming out from under the slab making a wet spot in yard, also leaked in a closet between two bathrooms soaking the floor. It was a cold water line feeding the front bathroom. Easiest fix was to use Pex and reroute through the attic This was all copper and the copper going under slab had a sleeve it ran through. There was never any loss of pressure or spitting air. Figuring out how the lines are run is a pita.
