That's what I was thinking Tom. Far more likely the plastic around the captive nut has become brittle and fallen away and the only thing holding it in place is the fact that it's still screwed to the thread.
When I recently removed my c-pillars one of the captive nuts was missing and the screw held in with sillicon adhesive.
If you remove your c-pillars for inspection expect to open a can of worms as with mine all the captive nut required reparing in way or another.