Slab leak ???

[scomea]

Howdy,
My son heard water running in the bathroom but no water was on the floor except for a little water under the 1/4 molding. The floor and wall was warm. No above slab damage.
I called a plumbing company and they located the rushing water sound using headphones/instrument. It’s a 17 year old house with copper pipe.
Because we had to leave town for 3 days, we turned off the water heaters, hot water circulating pump (2-50 gal) and house water. Everyone agreed that during this time, the excavator crew would tunnel 10’ under the slab to the leak.
We came back three days later, met the plumbers, turned on the water and no leak, no rushing water sound. Everyone was surprised. The system was pressurized to make it leak and nothing. We recirculated hot water through the system continuously attempting to open up the leak with no success.
Any ideas where to go from here?

Thank you

 

[Twowaxhack]

Either it was misdiagnosed in the beginning or the leak has temporarily sealed itself.

It can happen.

 

[Twowaxhack]

I’d put a pressure gauge on it and check for a leak, pump it up to 100psi.

 

[scomea]

Thank you. We’ll do that today and I’ll post the results.

 

[Duckbutter]

"The floor was warm..." - "located the rushing water sound using headphones/instrument. "

Sounds like maybe the hot water recirc was running.

If/when you hear it again, leave water on, shut off the recirc, listen.

 

[scomea]

Thanks to everyone for the comments. Plumbers came back today and proclaimed they found the leak. It was wetting the slab just enough for the technicians to locate the source. The pin hole was located about 3" below slab grade. A 3' section was removed and replaced. See photos. The house is 17 years old and is located in an area of hard water. I

Is there anything I can do to arrest the sulfide formation/manganese/black coating in the pipes besides epoxy lining the whole system? Should I take the black material to a lab for positive identification?

 

[Twowaxhack]

Maybe flux corrosion from when it was installed.

 

[Jeff Handy]

I would start saving up for a rainy day fund.

You might someday have to abandon some or all of the under slab water supplies and go overhead.

If you never need it, then you have fun money to buy a nice vacation, or a couple more guns!

 

[JG plumbing]

I'd definitely take it to a lab.

What line was leaking as in what kind of water was it supplying?