.5 inch copper pipe leaking at slab, .75 inch pipe from water heater enters
Ok, so I got this sorted eventually.  I cut a hole in the wall to expose the manifold behind the water heater, found the line feeding my leaking pipe, cut and capped it.  That resulted in stopping the leak but also cut the hot water supply to 2 bathrooms.  I then followed the wall with the leak to its end and cut another exploratory hole in the area where it abutted the hallway wall, and found more copper exiting the slab.  After some more fiddling, I discovered that the 3/4 copper coming out of the slab was feeding the bathrooms that were missing their hot supply, and that the 1/2 copper tee off of that line that dove back into the same hole was the pipe that led to the leaking section.  I then reattached the pipe at the manifold behind the water heater, cut and capped the 1/2 inch pipe in the hallway wall, replaced the drywall and borrowed a carpet cleaner to extract as much water as possible from my brand new carpet. 

So the answer ended up being that there were no fittings under the slab, the pipe was merely run under the hallway to the opposite wall where it poked out to a tee which was run under the slab down the wall.

Builders ought to have to file the complete plans with the city. Figuring out the plumbing and electrical runs in this place has been a nightmare.  Thank you all for your insight. It was helpful.