I am also having the same problem. It just turned on for no reason what-so-ever. We should try posting this in the technical forum tho, more people will look at it since that's where it belongs.
This is what I got from a Certified Volvo Mechanic online...
it doesn't help me any, but maybe you can interpret it better:
There are at least two technical service bulletins issued by Volvo related to this problem.
1) Customers may experience a lit PASSENGER AIRBAG OFF indication in the overhead console (S40, V50, C70, S80 MY07) or interior rear view mirror (S60, S80 MY06, V70, XC70, XC90) with a belted adult passenger in the front passenger seat. When this happens, the PASSENGER AIRBAG OFF indication will typically remain lit until the passenger unbuckles the seat belt or moves out of the seat.
This could be caused by vibrations on the bracket for the Occupant Weight Sensor (OWS) control module. Follow the attached instructions only after all other root causes (e.g. the hose from the OWS bladder must not be kinked, twisted or otherwise restricted) have been ruled out using the fault tracing in VIDA.
Beginning MY 2008, the PAD (Passenger Airbag Disabled) lamp illuminates when the front passenger seat is unoccupied (See Figure 2). Earlier vehicles, within the above range, that receive a total CAN or SRS reload or upgrade will receive the new logic. Some customers may misinterpret the new logic as a fault.
An actual SRS/OWS faults will always be accompanied by the presence of an SRS DTC and a DIM message stating either "SRS service urgent" or "SRS service required."
When fault tracing SRS/OWS complaints regarding PAD lamp operation, please make sure they are not due to the logic change described above. If they are, do not replace any parts. Instead, inform the customer about the logic change due to the software upgrade.
So, even though you're willing to give it a shot, you're going to need some specialized equipment and diagnostic electronics to give it a go. As these are fairly well-documented problems, I'm sure the dealer will be able to get through them with minimal time spent, and minimal expense to you.
And understand that this is coming from a guy who owns a shop, and in situations like this, would do exactly what I'm suggesting you do: bring it to the dealer, they have the proper equipment.
Thats what he said...I'm not sure how to follow it up either.