Kim Philby was recruited by Soviet intelligence in 1934, joined SIS in 1940, and by 1944 was head of Section IX — the section responsible for counter-intelligence against the Soviet Union. He held the position for years.
He was suspected in 1951, after Burgess and Maclean fled to Moscow and it became clear that someone had warned them. He was interrogated, forced to resign, and lived under a cloud for four years.
Then in November 1955, in the House of Commons, Foreign Secretary Harold Macmillan stated that no evidence had been found that Philby had betrayed the interests of his country, and that there was no reason to conclude that he was the so-called third man. Philby gave a press conference in his mother's flat, calm and slightly amused, and denied everything.
He went back to work for SIS, in Beirut, under journalistic cover. He confessed in January 1963 after being confronted with new evidence, and defected to Moscow before he could be brought home.
Three mechanisms kept him in place for nearly thirty years.
In-group trust. Philby was the right sort — Westminster, Trinity, a father who was a famous Arabist, the manner, the clubs, the stammer that people found endearing. This produces the halo of chapter 11 in its most consequential form, and the consequence is not that suspicion is dismissed but that counter-evidence is attributed to error: if a fact points at Philby, the fact is probably wrong.
Institutional motive. Once SIS had defended him publicly, and once a Foreign Secretary had cleared him in Parliament, admitting penetration meant admitting that the service had been comprehensively fooled and had said so on the record. Max Bazerman's work on motivated blindness describes what an institution does with evidence that would humiliate it. Janis's groupthink describes what happens to the colleague who raises it — and there were such colleagues, and they were sidelined.
And controlled candour. Philby volunteered his Communist sympathies as a young man in Vienna in the 1930s and treated them as youthful enthusiasm long abandoned. Admitting a small, verifiable, embarrassing fact inoculates against the larger one, because a person who has disclosed something damaging is assumed to have nothing left to hide.
The exposure, when it came, was documentary and external: Anatoliy Golitsyn's defection, and Flora Solomon telling an MI5 officer at a party that Philby had tried to recruit her in the 1930s.
Not one of the three mechanisms was defeated. He was caught because somebody talked.
Aftermath & Cross-Reference: Insider Threat Defence