xcitation. Without too much comment, there is a possibility nomenclature is not being understood. "On the right, 'side seat' (#4)", there was no belt attached. This may mean there was no remnant of belt attached to the seat frame. It would not mean there was no belt attached to a body, since without restraint, there would be no body remaining attached to the seat frame. BEA would ordinarily identify each seat by number, and RHS would be #2.
You say they identify "his mic". Not necessarily. They say "the mic". This could mean the ambient mic (one of, eg).
Either way, having found no belt attached to the seat, strictly speaking, it means that the belts were lost from their attachments (at impact?).