There are a ton of artcles out there that state that a battery that has a charge will not go dead if it is left idle unless it is not holding a charge or if the car is draining it.
Now if you left your key fob in/on/next to the car then there you go. pay the $400 and move on. If not then certainly you have a case.
Enjoying an E89 Z4, F30 320i and Fiat Abarth //
E85 Z4 (sold), E82 128i (sold), Fiat Pop 500 (sold)