You gave a couple of good clues.
You send you turned off the main valve and there was no meter movement. The reason for this is the leak is beyond the main valve.
You also said it stopped when you turned off the cold service to the water heater. This means your leak is on the hot water service.
OK, so lets reveiw what we know, The water leak is under the slab on the hot water service.
Now what we dont know is what we have to pay someone to diagnose for you and that is where is this hot leak.
Call a licensed plumber with an electronic slab leak detector. They will locate, break the floor and repair the leak, backfill the hole with sand and concrete to grade.
The Local Plumber
Tustin, California http://www.TheLocalPlumber.com