As you cannot see the valve that the water heaters run to they maybe leaking without your knowledge. Check the tank to see if you hear a hissing sound. That sound means there is a whole. Remove the top of the tank and listen for a hissing sound. You can also put a couple of drops if food coloring inside the bowl. If there is still coloring inside the bowl in a few minutes then there is a leak in the flapper.
The flapper is at the bottom of the tank. Turn of the valve then watch the meter to see if it is turning. If the meter is still turning and you have turned it off then there is a leak between the meter and the house. Look for any signs around the meter such as damp or muddy grass. If the meter has stopped running since you turned of the valve then the leak is coming from inside of the house.
Sometimes you forget to turn of the hose in your backyard and it may be dripping. After using a hose to water the grass always make sure that it is fully turned off as any little drip can cause the leak. If you have a pool in your backyard it is important that you check to make sure that there are no holes or cracks in the foundation as that can be causing the leak. Sometimes it is not easy to detect a water leak in your home, it is a good idea to then call professional Water Leak Detection in Scottsdale to get to the bottom of it for you.