When Smiley is talking to Connie Sachs, she mentions that Karla once had a mistress, upon which Smiley's eyes widen, and with an utterly surprised look, he asks "Who?" She then goes on to tell about how Karla also had a daughter from that relationship. It's highly improbable that George Smiley, who spent most of his life gathering every bit of detail about Karla, and even wrote "The Karla papers" (according to Saul Enderby) wouldn't know about such an important piece of information about his nemesis.