The most common cause for the air bag light to be lit is that the warning light bulb itself has intermittent contact. The bulb is supervised by the control unit, and any hiccups in the bulb holder or clock instrument connector will cause the control unit to fire a fault, and light the warning lamp.
This problem can be prevented by turning the warning lamp in the bulb holder, as well as pulling out the connector a few times. This will remove any corrosion on the contacts.
As mentioned above, a Porsche compatible OBD tool will reveal the cause for the alarm.
Cheers,
Tore