Where does the temperature/pressure relief valve drain line end? It may be plumbed to the outside, but if it ends at the base of the heater, try feeling the inside with a finger and see if there is any wetness.
Sometimes water DOES come up out of the ground. Or there is what we call a slab leak. For that one, make sure nobody is using water and check the water meter to see if any dials are turning.
As to plumbers, some will say that, but not everybody.