1. Is someone home all day if not could a neighbour be using your water to water there gardens.
2. Daily take a water meter reading to see if one day is worse than other.
3. Turn off all taps and take a meter reading and go out for the day and see if there is a change when you return.
4. If you do 2. and 3. it may narrow things down.
5. If you think someone could be stealing your water from the outside taps you can confirm it by eg. leave the garden hose attached to the tap coil it up the hose and make a mental note how you exactly left it if its moved slightly when you return you have you answer. Do that to all outside taps.