slab leak. Can't find the manifold in an old triplex

I just bought a triplex. The inspector failed to detect what turned out to be a slab leak in one of the units. We found the leak, hacked into the floor to fix it and discovered 60 year old galvanized pipes badly in need to being replaced. We rerouted the plumbing using new copper but the original lines were still pressurized. The plumbers intention was to cap off the manifold to prevent further leaks in the slab but he can't find the manifold. It's not behind the water heater or in any of the 6 walls he tore up repipeing and there are no plans showing plumbing schematics of this dwelling with the city because it's so old. This is the back unit of a triplex and all the water feeds from the city pipe through this unit and then into the others. Once the reroute was finished, he just capped off the hot water (I hope this makes sense, I'm not sure I even understand it) stating that the hot water pipe is usually the one that leaks and we should be ok. But it looks like before they even got the walls repaired there may be another leak in the slab close to the original leak. My question is, is there any equipment out there that can find a manifold behind poured concrete walls? If a plumber could find it and cap it off, would this solve my problem with slab leaks in that unit? If not, what is the likelihood of having continued cold water leaks in the floor? Finally what is the least invasive and most cost effective option to solve this problem?

Welcome Dr. Quack:
Your plumber needs to run the new copper pipe to the outside of the building and connect it to a new service line from the meter/street. That would eleminate any cross-connection with the old system and any future leaks.
It would be easy enough to cut a one foot square out of the floor, dig to the bottom of the foundation and run the new service line under the foundation. One bag of Sackreet would fill the concrete back in.
Glenn