Mystery leak

I've been asked to help solve the problem and am open to suggestions:

The leak is still a mystery.
To review what I've done numerous times over the past 8 months:

I've shut off the water at the house several times to check the water service line from meter to house. No leak there.

Then valve back on at the house, so everything inside is wet (ice maker off and valve off at back of house serving backyard irrigation/garage) and, consistently, a leak. Varying from 1.5 gal/hr up to 2.5 gal/hr. Strange that the flow varies.

Then I shut off of the valve going into the water heater; leak drops a bunch: down to .1 to .3 gal/hr.

Turn WH valve back on and start shutting off sink angle stops, shower valves and washing machine: flows vary slightly, but still getting a leak of 1.5 gal or more.
Note: I did replace the washer at leaking faucet at upstairs bath; helped a bit, but leak is still 1.3 to 1.7 gal/hr.

Test today:
with everything on (ice maker off): 1.8 gal/hr., then the next hour it was 1.5 gal/hr.

The mystery continues...

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