When you shut off CW to the WH means that you shut off all HW. Do you see any water arounds the heater if not then you have a leak in HW line somewhere, check out all the HW pipe leading to fixtures, check the stops and connections under the fixtures, have you noted any HW in the CW lines, if that all checks out you might have a leak in the wall, notice any wet floors. You sure it is HW leak, it could be your water service, shut off main valv in house and go outside and see if meter arrow is moving if so you have a leak on your srevice, I hope not let me know what you find.
Art retired plbg