The air bag module is held on the steering wheel with springs. That screw is most likely the screw that attaches the switch to the steering wheel. Has the car been in an accident? The only way that screw could come out is if someone screwed it in too tight and broke the bezel for the switch and it's finally fallen out with use/movement of the switch.

See pics

 

The CarFax report (provided by the dealership) showed no collision history of any kind. I can't see any body damage or repair work. But there are some odd things I've noticed about the car only after having it for a while. Both the hood and the rear hatch don't close uniformly, there's a larger gap along the drivers side on both. And there is a weird dent in the roof right at the rear hatch hinge area that I missed on inspection.

But that screw, I think, is almost certainly from the button panel on the left arm of the wheel. Now that its out, the panel is noticeably loose where it wasn't before. Is there anyway to get at the back of that panel without having to mess with the airbag assembly?

 

I'm not familiar with that model year, but take off the negative lead from the battery if you do start taking the steering wheel apart, of course.

 

If the hood and rear hatch are out of alignment and you have a dent, I'd say it may have got a hit and it may have deployed the air bag.

Sometimes when an air bag deploys things that are in the vicinity get damaged. If the bag blew and the switch got damaged, then the switch would be replaced with the bag.

It may also be that the radio switch quit working for what ever reason and was replaced.

The only way to get at the screws is with the air bag off the steering wheel. Disconnect the battery cable, turn the ignition on and hold the brake pedal for a second. Then you can start taking it apart. You use a small screwdriver to pry the spring out of the way and go around in a circle.