As an insurance adjuster, let me pop in and comment as well as ask a few questions.

Was this an insurance company direct repair facility or was this a shop of your choice?

Did you get a copy of the repair estimate written by your insurance company and the final invoice from the body body shop? If so, does it mention anywhere to disable and reconnect the air bag system. If major electrical work or welding is being done, this my be required.

There should be no excuse for body filer dust to be in your car when you picked it up. I would take it back to the shop to have them clean your car. This may have been an accidental oversite.

As mentioned, depending on what is being done to the car, it may have been necessary to disconnect the air bag system to safely complete the repairs. Again, I would return the car to the repair facility and ask them to look at the problem.

If the shop denies responsibility for either issue, contact the adjuster/appraiser who looked at your car or handled your claim file with your insurance company and seek direction.

If there appears to be no reason to have disable the air bag system and there is nothing on the paperwork indicating the shop tampered with it, you may need to have this corrected at your expense, and I would suggest having the dealer handle this issue. I would also request they locate the reason for the fault code and that the dealer put it in writing. If related to the loss, either the insurance company will handle as part of the loss, or the repair facility should reimburse you.

As for an air bag deployment, what you see is what you get. If the bag had deployed, you would have seen the bag drapped out of the steering wheel or dash board. The front air bag is designed not to go off on a side impact. I would look at the main air bag control module being bad from an accidental or coincidental electrical short, or a portion of the system being disconnected.

By the way, as cars age, things do happen. Don't be shocked if the problem is completely unrelated to the repairs.

Handle this with a good attitude. Most shops want to keep customers happy and they will usually bend over backward (within reason) to keep you happy. A good shop will handle these issues in a professional manor.