I had a similar problem with a battery in a new car that would go completely flat within days.
The dealer replaced the battery like for like and all seemed well for a month, then the battery died again.
First dealer had closed so went to a different dealer, they again swapped like for like, and a month later it happened again.
So they looked again and noticed that the battery was the wrong model, it was the battery for a petrol model rather than a diesel. So they fitted the correct battery and everything was hunky dory and I had no more problems for the five years I owned the car.