What I like the most about Probable Cause is the fact that, since the begining of the show, Beckett has usually been the one driven by “the evidences” . Castle was always the one to elaborate weird and fanciful theories. Then when all the evidences point to him, she doesn’t want to trust them anymore. When she sit on “his” chair on a early scene in that episode it’s like she’s already trying to see the world like him, with his eyes. That’s why, when he tells her that insane story in his cell about 3xK being back, despite the fact it may sounds crazy, and even if there’s no one on the camera footage, she totally believes him. Because for him she’s willing to choose the story over the evidence.