Get someone to locate the line under the slab. Electronic Locating equipment should be able to trace the line unless it's plastic. Maybe prints from the building department/ permit could help. Exposing the service line outside the house foudation will help get a starting location and direction. Then drill a series of holes through the slab, a few feet apart over the manifold but not through it. Hopefully one hole is a gusher instead of a slow weeper. Then use a concrete saw and electric hammer to remove a small area of slab. Hopefully you are able to find the manifold. See which way the water is coming from through the gravel under the slab and remove more concrete untill you find the leak.
Manifold is in the house.
One thought I had was to disconnect each line, fashion a fitting on the end and pump air into each, one at a time. Figure I might be able to identify one that isn't holding the compression.
