I would have thought that if you took the seats out, with airbags in, then they wouldnt actually fail it as the seats in your car (sparco etc) dont have airbags in the first place.
Hard for me to describe what i mean but, i would define 'missing' as an OE steering wheel, without the airbag inside it, not a different steering wheel that didnt ever mean to have them in there in the first place, not having the standard one in there.
I would have thought it sort of means;
If it has gone off, must be replaced and ready to fire again.
If someone has changed to a R32 steering wheel for example (which has space for airbag) you couldnt have an empty space behind where it should be?
And call Me/friendly MOT testers taking £20's idiots/fools/irresponsible but regardless of the law, its always going to happen, and now, probably even more..