If both occupants were belted, then I think the most likely explanation is that there was third person as speculated earlier (a driver) who got out. But I think most likely the claim about the person in the back being belted will later be found to be false. Maybe they found the buckle in the receiver but the belts themselves almost certainly burned up in the fire. Finding the buckle in the receiver doesn't mean the occupant was belted. I've seen people leave their back seats buckled all the time for a number of reasons: to keep the buckle from rattling against the side for example.
Mike
Mike