I think William could have also been restrained and well-hidden if he was contained by the usual vehicle front passenger seat belt. He may have felt very important being 'allowed' to sit in a front seat. Especially if it was some kind of 'special' vehicle, not just a regular car.
I had often wondered, after that great article was written where the reporter walked Benaroon Drive with Jubelin, if they have some CCTV showing a vehicle with the passenger seat extended, but no person visible in the passenger seat.
Yes I agree , like a white van ? Many years ado when my middle daughter was around Williams age we had a few incidents of a man in a car trying to engage children on their walk home from school . My daughter was obviously to young to be walking alone but was relaying to older son about this and little one pipes up and says " I'd go with them if they had good lollies"