This seems to be quite a common problem now, not specifically on your particular car but generally, probably due the the ammount of electronic equipment on modern cars.

The problem is probably being caused by something which is continuing to draw current from the battery when the car is not being used. Check in the dark and make sure there are not any lights left on, in the boot for instance, if nothing found connect a multimeter on the amps range between the battery positive terminal and the connector which you will have removed, this is with everything turned off, a reading of a few mili amps is normal but it should certainly not exceed 100, that would flatten a good say 40ah battery in 400 hours, other currents and capacities pro rata.

If you do have an excessive current draw, try removing the fuses one by one to lacate the culpret. While you are using the meter connect it across the battery with the engine running, if its charging correctly it should read at least 14 volts.