You may have a water leak. There are several ways to check for a leak. One way is to check the indicator on your water meter. Make sure you have nothing inside using water when you do this. If the indicator is turning, then water is flowing through the meter. This could indicate a leak in the water service line between the meter and your home or building. Or it could indicate that water is being used inside the home or building.
To determine if you have a leak in your water service line, close off the main valve to your house and look at your water meter. If the indicator on the meter is turning, this is an indication that you have a leak in the pipe between your water meter and the main valve (though it could also mean that the main valve is not shutting off the water completely).
If the indicator on your water meter was turning but stopped when you shut the main valve, this indicates that something inside is using water. With the main valve open (and the indicator on the meter turning), turn off the valve to individual fixtures (toilets, sinks, etc.) one at a time. Check the indicator each time you turn off a fixture valve to see if it stops turning. If it does, the last fixture turned off is the culprit!
Toilets can often be the cause of a "mystery" water leak. Open the cover on the tank. If the water level is above the overflow tube, there is a problem. The water level should be approximately 1/2- inch or so below the top of the overflow pipe. Adjust the float level, so that the water is turned off at that level. If the valve controlled by the float is leaking, it may need to be replaced.
Toilet leaks can also be due to a leak at the flapper valve that lets water flow into the toilet bowl. To test for this, put a drop or two of food coloring into the tank (not the bowl). Wait for about 10 minutes and see if the colored water shows up in the bowl. If it does, your flapper valve may need to be replaced.