One problem related to air bag light on has been reported for the 2008 BMW 335. The most recently reported issues are listed below.
Passenger airbag seat mat occupancy sensor fault. The problem began whenever a passenger would sit on the seat, the airbag light would go on. After some time passed the light stays on permanently. I noticed recalls for this problem in previous years. Upon researching, it seems to be a wide spread problem that just isn't reported. It leads to many just fixing it themselves, or just buying bypasses to avoid forking out over $1200 to get it fixed. The problem doesn't seem like a normal wear and tear situation, and needs to be addressed.