The Airbag Light can be on just because you Unplug and Replug the Seatbelt Receptacle, or it can be on because the seat you removed had Integrated AirBag or Weight Sensor (my 987 have those, I don't know for 996, probably not)
When I replaced the seat in my 06 Boxster S for the GT3 Seat I needed to tell the car to ignore the Weight Sentor and the Thorax Airbag in the seat because the GT3 seat don't have any..
If you like to be aware of what happen with your car and are a little on the DIY side, get yourself a Durametric, you'll be able to see exactly what's happening in your car's Brain, you'll be able to easily Turn off (Reset) that light just like the dealer would do it, and you'll even be able to manually disable some airbag (for track purpose), and you'll be glad to be able to disable the 'ding' sound when your seatbelt is not on (witch will prevent the ding-ding-ding when you got the racing harness on because the factory seatbelt is not buckled up), and even put it back on for street driving..
I have one and I never regreted the investment..