sleeper
n. A spy who emigrates to another country, spends many years living a normal life under an assumed name, and then begins spying once he has worked his way up to a sensitive government or corporate post.

Example Citation:
"Pentagon officials told a congressional committee last year that the Chinese are planting 'sleepers'—deep cover agents—in U.S. high schools with the intention of getting them into leading universities such as the Massachusetts Institute of Technology, and from there into high-tech industries."
—Jeff Sallot, "Spying comes in from the Cold War," The Globe and Mail

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