havasu

Here is a silly question but have you attempted to place a few drops of dark food coloring in the toilet tanks, to see if the water is migrating into the bowl without flushing?

Next question...T&P valves as Redwood is speaking about is on most water heaters. Did you say you had one at all, or you said you did and there was no water seepage going into the drain?

Lastly, Isolate all exterior sprinklers, if you have them. Disconnect, shut down, whatever is necessary to stop water from any irrigation.

Basically, what I am trying to do is isolate all your lines, similar to diagnosing a short in a wire, so you can fix this problem.

Redwood

The fact that the leak is indicated intermittently on the meter rules out all the things such as leaking pipes and other leaks that won't stop.

It does leave open possibilities such as leaking toilets, relief valves, and irrigation leaks that a valve on a timer may control.

Toadfish

All the toilets are shut off, no lawn sprinklers. Relief valve on HWH was dry. If there is another relief valve buried in the wall or ceiling I couldn't find it and it didn't dump to the exterior of the house.
And having it tie into the waste line is a no no.
I would have thought after putting 100psi of air thru the system I would have been able to hear it.

I also did the dye in the tanks, no leaks.

havasu

Since my head is nearly bald from all the head scratching, at this time I would contact the water department. Explain all you have done and insist on a new meter. At least that would be ruled out. Since they usually have a helpful customer service department, they may also spend a bit of time to help you look in every direction possible.

oldognewtrick

If you pressurized the system, was it holding pressure or falling?

nealtw

Have you checked out the pool next door. People steal power why not water?

havasu

This does happen in LA everyday. Good thought!

Toadfish

falling
No pool.
Unless they tunneled underground and made a connection in the slab, I see no hose running across the lawn.

havasu

If the water pressure was dropping when pressurized, there is no doubt you have a leak, and with all your snooping without locating the source, it is time to get a leak specialist out there.

It sounds like it will be either under a concrete slab or adjacent dirt, and will not go away. As a matter of fact, the leak will eventually get worse.

nealtw

Some builders up here will tape a line to the house next door during construction, when they have built both houses, when there done with it they cap and cover it. Some smart homeowners figure this out and hook up sprinklers and such. So I really wasn,t kidding.