A toilet leak may not occur until the water drains to a certain height in the tank. Add some food coloring to the water in the tank and let it sit for a half-hour. Then check the bowl to see if the colored water has leaked into the bowl. If your faucets have old gaskets, it can take a lot of pressure to shut it off fully. With different people using the faucet, it may not always be completely shut off.
Sprinkler heads can sometime stay open and leak water onto the lawn.
Each sprinkler head uses between 3 and 5 gallons per minute. A station on a sprinkler system could possibly leak as much as 90 gallons per minute depending on several factors. That can add up very quickly on your bill. These are just a few suggestions to look for water loss first. If you will check these things first, it may save both you and the City time and money.