battery died with no warning

my car is a 2011 328xi

My vehicle is in the shop and I am getting a 600$ bill for checking the battery and getting a replacement installed. The car went in for a general service check 3 weeks ago and the battery was supposedly checked and supposedly passed inspection. There was no incident of leaving the lights on or running the radio while the engine was off. Our driving habits have not changed...typically the car is driven to the cottage (240 km mostly highway) and driven back (another 240 km) each weekend. During the week it generally sits idle. The battery died on the sunday after the friday drive to the cottage. The engine did turnover but would not start, no indicators of any kind were displayed. I have two questions:

1. there was never a battery warning indication, the car went from fine to would not start. Is this unusual when a battery dies? I assume there should have been some kind of warning...

2. Is it reasonable given our driving habits for the battery to pass the inspection and then die within 3 weeks? The dealer claims the battery can go from fine to dead in a few days.

I suppose a third question is are our driving habits bad for the battery... I should also mention the car was parked indoors the entire time both in the city and at the cottage.

Thanks for any feedback

battery died with no warning