Nationwide, the commonest cause of water loss is a leaky toilet tank flush valve. They are most often silent, although a large leak will result in a constant "toilet running" noise. But even a small, silent leak will result in the loss of hundreds of gallons a month.
A simple do-it-yourself test will let you know if you have one of these sneaky leaks. Carefully remove the lid of the toilet tank and place it on the toilet seat or floor to avoid damage. Put 3 or 4 drops of food coloring, Easter egg dye, or cake dye in the toilet tank water and replace the lid. Check the water in toilet bowl an hour later. If there is a dye tint in the water, you have a leak.
Fixing the leak is often as easy as replacing the flapper on the valve.