In 2367, an explosion in the Enterprise warp core leads investigators to discover a Klingon (J'Ddan) on board the ship involved in smuggling secrets to the Romulans. J'Ddan admitted to his espionage, and hinted that there were co-conspirators on board the Enterprise as well. Simon Tarses, who occassionally administered a medicinal hypospray to the Klingon traitor, was extensively questioned by a Starfleet board of inquiry, which uncovered that Tarses was a quarter-Romulan (not a quarter-Vulcan, as he had falsely claimed on his Starfleet entrance application). His falsehood threatened to end his Starfleet career. Tarses was originally from the Martian colonies. (TNG: The Drumhead)
Note: By TNG: The Drumhead, it was established in dialogue that Simon Tarses was an enlisted crewman who had not gone to the Academy; therefore, he wore a blank collar. This lack of rank insignia for crewmen and noncoms was used throughout DS9 and VGR, although chief petty officers were assigned different insignia.