Many years ago I use a sonar type piece of equipment to find a leak under concrete. The closer I got the louder the noise in my head phones. You have to have water flowing so the sonar picks up the noise.
I would check with the manufacturer of the tubing to see if they offer a splice kit for under slab. Usually these kits have a special crimp ring that will not be damaged by the minerals/chemicals in the concrete itself. It will not be a fun job and you will find you might have to have a large area to work in for the splice.
OR
Depending upon how the system was piped (how many loops) You might be able to disconnect the problem loop and raise the water temp a bit to compensate.