Once you determine the location where the pipe enters the slab area, and
where it comes out of the slab, draw a line on the slab and drill a few half
inch test holes to see if the dirt is dry or muddy. Or if possible, just
abandon the pipe under the slab and run a new one
Some Menards sell a cheap moisture meter for measuring wood or
concrete moisture content, where it is wettest would be the leak. If
it is old copper it might be worth abandoning as acids in concrete
have made many radiant heating systems get leaks. A microphone moved
along the pipe area might alow you to hear it leaking
Here's a link to message board thread where a problem like yours is
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