How did Cao Cao know that Sima Yi was going to defect?

Not a majestic gait, but an eagle-eyed wolf looking at an eagle-eyed wolf. What is never cruel is a sharp-eyed wolf. Well, it is said that a wolf can stand still and turn his neck back to 180 degrees. Sima Yi has this ability. It is said that such people are reincarnated by wolves, which is one of the reasons why Cao Cao distrusted Sima Yi in his early days.