His guard was never down. He knew he was a target long before talking to the FBI, because the calls he had with Kislyak leaked to media before Trump was even inaugurated. Also, fucking Obama basically telegraphed it by warning Trump about hiring Flynn just two days after the election, which I'm sure he relayed to Flynn since he hired him anyway. Then, day four as NSA, McCabe all of a sudden wants to send some agents over to have a "totes friendly chat" and no need for lawyers. Nah, Flynn saw that shit coming ten miles away.