You most likely have a parasitic drain on the battery from a defect within the car. These can be troublesome to track down.
A few things to check, there is a little flap on the ignition barrel, when the key is removed it should close. Check all auto functions are switched off. Make sure you double lock the car when left unattended, even when in a garage.
In the meantime, a CTEK battery maintainer will help keep the battery fully charged.