Dropped my car off at the dealer today, and it was fixed in about an hour. On the service write-up, I found a comment about SIB630610. Turns out that is a somewhat common issue and there is service bulletin for it. Below is the info I found online. Lucky or unlucky, my car was built on the last day of the covered range.
LEDs in Rear Lamp Inoperative
E90 (3 Series) from August 31, 2008 to February 2, 2010
The LEDs (Light Emitting Diodes) in the outer left or outer right rear lamp are inoperative. Affected are the
LEDs for the parking lamp/tail lamp or the turn signals. No Check Control message is displayed.
Corrosion of an internal connection within the tail lamp
Replace both outer rear lamps (left and right), even if only one side is inoperative. Refer to Repair Instruction
REP 63 21 180.
All lamps are to be retained intact by the dealer, awaiting either:
A parts 1. return request to send parts to BMW
An indication to scrap the parts based on information provided in the dealer's scrap report, at which time the parts are to be destroyed to prevent reuse."