I had a similar incident with the right side bag falling off my R11GS when the mounting tab broke.
The dealer claimed it was as a result of an accident and pointed out scratches on the left cylinder head. When asked, he refused to even look at the bike to explain how a left side parking lot tumble damaged a right side bag...
I asked to be referred to a higher authority (BMW), and he stated that the issue was between me and him, and that BMW would believe his word rather than mine.
This was one of a string of poor service experiences with various BMW dealers and BMW North America. I have now sold both BMW motorcycles. In fact, I just bought the wife an '03 VFR ABS, to add a bit of Red (actually silver) to my two Kawasakis....
Actually, the BMW thing did have a happy ending. In the last of an ongoing series of emails to get my dissatisfaction with service and reliability resolved, BMW Customer Service recommended that I switch to another brand.
I did exactly that, and haven't looked back since: our two BMWs were swopped for a Concours, a ZX12R and the VFR, as I have always had good service from Honda and Kawasaki. Next down will be the wife's 330xi cage.