I have had nothing but trouble with the passenger airbag light on my 300c. It used to randomly light and the chime would sound every few minutes. They recalibrated several times, replaced the switch, etc. For a while it was just the light in the center console and mostly when we hit a bump. After about 8 trips to the dealer, it now seems to illuminate/deactivate due to a "negative weight code" when someone in the back seat touches the seat rails that are bolted to the floor. My dealer, Southworths in CT, initially worked hard to solve the problem (hard to argue that the light and chime should activate every few minutes when the front and back seats are empty). Now that they think it has to do with a rear passenger putting his feet on the floor in certain places, they claim it's normal and that all cars will deactivate the airbag if the rear passenger touches the floor like that! Now they want to charge me for the loaner they gave me because they say this is normal and that there was no warranty problem! I am ready to start the lemon law process to make them take it back or fix it...if the airbag misbehaves I think the car is unacceptable.
You can probably get the random deactivation fixed with either a recalibration or a new sensor. If there is no fault code, they may well have this negative weight code problem. I would certainly advise steering clear of Southworths if you want this fixed without a hassle.
We generally have a driver and someone in the rear passenger seat. Nothing on the front seat, nothing touching the seat on the side or underneath.