Apparently the interior is installed along the Boeing assembly line. At a minimum, the person attaching the interior panel over the door plug should have seen a lack of bolts. At a maximum, the door plug would have been removed at some point to aid insertion of equipment, and then should have been re-bolted.
Also, at this time, the 737 interiors are worked on some more. Sidewalls, seats, lavatories, and galleys are installed along the assembly line at this stop.