According to Webster, sleeper can refer to (4) someone or something unpromising or unnoticed that suddenly attains prominence or value or... (7) MOLE entry 2 sense 4  ie: a spy (such as a double agent) who establishes a cover long before beginning espionage broadly : one within an organization who passes on information Etymologically: Sense of "something whose importance proves to be greater than expected" first attested 1892, originally in American English sports jargon, probably from earlier (1856) gambling slang sense of "unexpected winning card." Meaning "spy, enemy agent, terrorist etc. who remains undercover for a long time before attempting his purpose" first attested 1955, originally in reference to communist agents in the West. Ain't dictionaries fun?