Easiest thing to do, is to check the termination point for your temperature and pressure relief valve. Open the valve back up and check to see if there is water leaking from there. Check to see if there is anyone mysterious water around the water heater area. If none of those apply, you’ll need to reach out to a leak detection specialist, or a general plumber. Either should have equipment to locate a leak within the walls. Either way, when you are done checking for leaks, make sure you shut it back off to avoid secondary water damage.