Unexplained high water bills can be difficult to detect but the approach of elimation is best. 

First things first. Check the watermeter for movement.
The best is to take a photo before bed and another photo first thing in the morning and then compare the figures. 
You can also moniter it over a period of 60 minutes and compare the two readings. 

On the standard french meters, the black numbers are the cubic meters and the red numbers are liters.  On the example image the reading is 3864 cubic meters and 231 liters. The red dial below the red numbers indicates one liter if it went around one time.  The last red digit would then tick over once to 232 liters. When the red digits arrives at 999 liters the last black digit tick over once.

If you do not know what to do next, a leak detection company has the skills, knowledge and tools to find the problem without destruction and propose solutions based on where the leak has been found.

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If you want to try to narrow down the search yourself, here are a couple of tips. 

Usually there is a valve that closes all the water in the house. If this valve is closed and the water meter is still turning, that means the leak is in the garden somewhere.

If it has stopped turning it means that the leak is in the house.

Two things to check before calling a leak professional is:
a.  The toilets
b.  Pressure release valve underneath the boiler

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