A low FOB battery is pretty cheap to replace and can cause many "odd" symptoms. If it happens again before you replace the FOB battery, place the FOB into the slot in the glove box. That then over-rides the FOB battery, forcing "recognition".
I received the same letter and, since I have been having intermittent air bag warning lights, I went in. The checked the codes and found that there was a suspect sensor but didn't get the code pertaining to the letter. So they charged me $140 for checking the codes!!! I've appealed to GM.
A couple of years ago when the light came on and off I checked the wiring under the driver's seat and found a loose connection. But the light continued to randomly come on and off. The I replaced the car battery and the light problem stopped . . . until recently when it came on after "horsing around" on the way to get new tires.
Mine is a 2006 C-6 "vanilla" coupe.
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