I got a call from a client that has a water leak on a 3 story residential home. About 2.5 gal per hour leak. I looked at the water meter and saw the flow meter turning, so I shut the main inside the house and it stopped. So no leak on the main. Reopen valve. Checked and shut all of the toilet supply valves, meter still turning. No faucet, hose bib or washing machine leaks leaks. Check pop off on HWH no leaks. No humidifiers on the hot air system. No water softener. Listened to the PVC stacks for water running, none. The house is part on slab and part unfinished basement. No leaks seen. It doesn't appear from a visual inspection that any of the supply lines go into the slab but rather up thru the wall. So I doubt it was leaking into the slab. I shut the main at the meter and dropped the pressure and then reopened and the flow meter is rock solid, no more water flow from flow meter. What the? I'm scratching my head here.