In the Sherlock Holmes story The Adventure of the Second Stain, the stolen item is a letter from a European statesman.
The letter is a confidential document addressed to the British prime minister by a foreign ruler. If published, it could create a serious international crisis and possibly lead to war. The British statesman asks Holmes to recover it discreetly before its contents become public.
The document was taken from the bedroom of the diplomat who received it. Holmes investigates the room, the visitors, and the political pressure surrounding the theft. The case ultimately involves Eduardo Lucas, a French agent, and a woman whose personal crisis drives the central deception.
The story first appeared in 1904 and was collected in The Return of Sherlock Holmes. Readers sometimes remember it as a “stain” case involving physical evidence, but the title refers to an earlier related crime and the diplomatic letter is the immediate object of the investigation.