Hi Miranda. I believe this is a fairly common thing on Mondeos. The Air Bag system and the Seat Belts have a common warning light- problem with the Air Bag(unlikely) or problem with the Seat Belt pretensioners- these are located by the seat belt catches where you plug in the belts.( they lock the belts instantly in the event of an accident that triggers the Air Bag) brings up this light. Under the seats are the wires to the tensioners-these can get kicked by rear seat passengers, Check these wires are in place-give them a bit of a tug to make sure-usually solves the problem!! The light may reset itself and go off after some time, or you can get a garage to reset it. incidentally, it wont fail the MOT if its on- I would ignore it and carry on as usual- I doubt you have any real problem.