I'm going to attack a similar problem this weekend and have been reading this thread. One thing I plan to do is to shut off the supply to the hot water heater to see if I can isolate the circuit where the leak is --- hot or cold. I'm not sure what I'll do with that information yet but it may give me a clue as to where to look next.
Just to throw out another idea, I once worked on an "unable to locate" leak in a home and after hours of crawling around, I discovered a small pinhole leak in the clear poly tubing leading to the fridge ice maker. It appears the constant vibration against the wall rubbed the hose in such a way the leak could not be seen, and it sprayed directly on the wall, down the baseboard, and into a seam in the concrete.
I suspect my ultrasonic leak detector would have located a leak after pressurizing the piping. Unless like you mentioned its a leaking PRV with the outlet buried in the ground somewhere.
That's the only thing I can think of. The interior piping layout just doesn't appear to be in a slab.
I'm sorry Mr. Redwood for "barking" but since there are two people on this thread with a similar problem, I was just throwing out various ideas, because many people have various leaks, and it may just help someone else who have come into this forum for help.//