Took mine on a 20 minute drive, switched it off, all normal.
10 minutes later when i switched it back on the SRS light would not go out. After several attempts at switching it on and off it still would not go out.
Took it to the local Indy who put it on STAR and had it off within 10 minutes. He said something about the "airbag system having a sudden fall out with the car and not communicating with it".
He reset it and a year later iv not had anything back.
So even mine came on out of NOWHERE as you put it, but it wasnt a massive thing to get it reset.