We have had exactly the same issue (which has occurred on a couple of occasions). Our car was with the local Seat Dealer for a couple of days. At first, they could not find anything (there was no
OBD logs of these alerts occurring). They then repeatedly started and ran the car for a short while. Out of the blue, it then did the same for them. Again, they checked the logs - nothing which meant there was nothing more they could do. They have assured us the airbag system is working fine and that this seems to be an anomalous alert. They've asked us to keep monitoring things.
Another gremlin in the software of the Mark 4.