The plan showed a room without an entrance. Visiting researchers called it the secret office, and the joke survived long enough to sound like history. Clara found the same rectangle while preparing an exhibition about the records building.
She asked for the earliest maintenance file. A requisition for shelving mentioned an open recess; a later fire inspection called it a cupboard. The plan had been copied between those two changes, after a wall was drawn but before the drafter added the door symbol.
Clara nearly discarded the puzzle as a copying error. Then she noticed a clerk's initials beside each revision. The same person had recorded every small alteration for thirty years. His final annotation, written shortly before his death, asked that the plan be corrected before anyone relied on it.
The exhibition displayed the unfinished plan beside that request. Visitors still stopped at the room with no door, but the caption led them toward an overworked clerk rather than a hidden conspiracy. The real mystery was how his warning had stayed attached to the page while so many readers overlooked it.