George Smiley had liked the man and now the man was dead. Suicide. But why

An anonymous letter had alleged that Foreign Office man Samuel Fennan had been a member of the Communist Party as a student before the war. Nothing very unusual for his generation. Smiley had made it clear that the investigation - little more than a routine security check - was over and that the file on Fennan could be closed.

Next day, Fennan was dead with a note by his body saying his career was finished and he couldn't go on. Why Smiley was puzzled ...