In a pipe? In a tank? Give us some more info, mate.

Under a concrete slab, in a private residence.

how heavy is the leak? is there an underground spring? a river nearby? is it from your drain or feed lines?

For drain lines you can add dye to the water as you send it down the drain. For feed line just ask you water company for a leak detection. The dye will work for most water flow problems

Dig the slab up.

When the slab was poured, the water pipes had to come up through it somewhere. That's probably where the leak is. The rocks in the ABC will pierce the pipe, with time. I'd look at the water line that runs from the water heater into the slab. It gets thermal cycled by the hot water, with use.

Late at night, when everything was quiet, I'd listen for the water running and follow the sound.

This happened to me.

You can hear leaks like that with a ultrasound listening tool.

not so smart,

I believe you are thinking of the Leavenworth in Kansas, not Leavenworth Washington...

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Just my $0.02...