Not all leaks leave a puddle on the floor. The sneaky leaks manage to drain away without ever leaving a sign. Toilets can leak a surprisingly large amount of water - over 10 cubic meters per day! To diagnose this problem, pour three drops of food colouring into the tank. If you see dye in the bowl 15 minutes later, you have a leak.
Get familiar with your water meter and learn how to find the Low Flow Indicator - it can help you detect leaks indoors and outside.