The records room

An original fiction passage set in the nineteenth century.

A dry rustling came from the records room as the courier crossed the deserted corridor. The building's keeper had gone to fetch his keys. She lowered herself beside the door and examined a narrow trail of disturbed dust leading beneath it. Next, she crossed to the adjoining window and looked through a loose pane, scanning the shelves until a bundle of papers stirred again.

Give two details showing how the courier searches for the source of the rustling.