I think I may have this same issue with my wife's '11 Outback.
Typically when there is no passenger there will be a "Pass Air Bag" and an "OFF" light illuminated.
This past weekend on 3 occasions I started her car and neither of those lights illuminated. Instead I was treated to the airbag light on the dash, the 'passenger seatbelt unbuckled' light, and the damn dinger dinging every 20 seconds...
On two of the occasions I could get the situation to clear by actuating the main On-Off switching apparatus, but it had to be after 10 minutes - 1 hr of driving. After things warmed up???
PITA