We have now done a more thorough check. There is no passenger seatbelt warning at all. However, if the driver's seatbelt is undone a warning sounds, and a seatbelt sign flashes in the rev counter gauge, as soon as the car gets to 20mph. Could it be that when you thought the warning was sounding for the passenger it was because the driver had not done up their belt? Hope this clears it up for you.