Double agent
A double agent or mole was a spy recruited from within an agency by another agency to report on activities thereof.

In 2375,  of Species 8472 impersonating Boothby asked Chakotay if he was a Romulan double agent, as he had just arrived from the Neutral zone and planned to enter Logistical Support, a secure area, with a holo-imaging device. ()

Also that year, Koval acted as a Starfleet mole for Section 31. They planned to insert him into the Romulan Continuing Committee following the removal of Kimara Cretak from consideration for a seat thereon. ()

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Illegal occupations