When water gets underground it can snake a long way before visibly bubbling up. Could be his runoff or from someone else even higher. Had something like this before and I'll be damned if it wasn't water from a drainage ditch funneling through either a dead tree root canal or a hell of a chipmunk den.
ETA- If it is a leak, nondestructive leak location is cool. They turn off your water, drain the line, pressurize with air(?), and use sensitive equipment to listen for the hiss of escaping gas. Shockingly accurate. A friend had this done for a line under his slab and was dead nuts on. Saved him a bundle on foundation demolition.
there is another way to fix it besides busting up the foundation. Back when I was younger, one of dads neighbors built a house. They finished it out, hooked up the utilities, turned the water on, and it sounded like an outside faucet was running. Water started seeping up outside of the foundation.
The guy and a friend did the rough in for the slab and dad told him he shouldn’t have used fittings under the slab. Neighbor said he didn’t an used soft K copper throughout and protected it with sleeving anywhere near concrete.
Turns out, lightning struck the site just after the concrete was poured. It hit the copper and busted it under the slab. Didn’t know it until house was built. His neighbor just casually said lightning had struck and knocked out some stuff at his house. This was same time House was being built.
instead of busting up the concrete on a new house, they bored out about a 1.5 inch hole diagonal lay down through the edge of the foundation behind the first spot where the lines came up from under the slab. It was behind the kitchen sink. They then did the same down the home behind where the lines came up in wall again. Bored a hole there.
Then they ran a line outside of the home from one header to the next and bypassed the pipe that had split from The lightning. Put sleeve on the pipe running through the hole and then put a stiff foam surrounding it. Buried the outside bypass line a couple of feet deep.
No concrete busting. Was it up to code? Nope. But he lived out to where it wasn’t inspected anyway.