I have to use my indoor cut off valve to keep the hot water from running out of the tank. Note the hot water faucets were not inuse any where in the house.
Do you think it's the hot water tank or a broken hot water line some where in the slab?
Note the cut off valve is on the cold water entering the tank.

Oh Oh slab! Yes, quite possible a leak in or under the slab. If everything is turned off and you are using water you have a leak. If the tank was leaking that much, you would see water leaking somewhere.