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Exposure of either one could spell disaster. He covered the tracks of his double infidelity with precision and care. Every few days, he would send a disguised message to Leila, and commit adultery in a different Copenhagen hotel; every four weeks, he would make his way to an unremarkable flat in a boring Danish suburb, and commit treason. Over the course of a year, he established a system of evasion, eluding both Soviet surveillance and the suspicions of his wife. His relationships, with both Leila and MI6, were deepening. He felt safe. Which he was not. One winter evening, a young Danish intelligence officer was heading home to Ballerup when he spotted a car with diplomatic number plates parked in a side street, far from the diplomatic enclaves. The young man was curious. He was also trained, and mustard keen. On closer inspection, he recognized the car as belonging to the Soviet embassy. What was a Soviet diplomat doing in the suburbs, at 7 p.m. on a weekend? A dusting of snow had fallen, and fresh footprints led away from the car. The PET officer followed them for about 200 yards, to an apartment block. A Danish couple were leaving as he approached, and obligingly held the front door open for him. Wet footprints crossed the marble floor to the stairs. He followed them to the door of a flat on the second floor. From
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Ben Macintyre (The Spy and the Traitor: The Greatest Espionage Story of the Cold War)