There is a weight difference in whether the airbag deploys or not. I'm pretty sure is 90 lbs. Despite your seat belt gong going off, I don't think your passenger airbag is associated with it. You'd have to look to see if the Passenger Airbag disabled light is on next to the SOS button.
'13 35i M-Sport X3 | JB4 Stage 1 | BMS Intake | H&R Springs | Gloss black grill | CF Wrap and Coded by me | Recaro Child Seats
'15 M3 DCT AW/SO | JB Stage 1 | RKP Diffuser | P3 | M Perf Spoiler | iND Exterior Trim | Recaro Child Seat