Upon greeting Ryan and Esposito at the crime scene, Beckett asks about the results of their sergeant's exams, to which they shush her in fear of jinxing it. She does the same thing to Castle in "Hollander's Woods (2015)" when he mentions the forthcoming results of her captain's exam.
Alexis and Martha mention that Castle's depression about the separation from Beckett has manifested itself in, among other things, Dolph Lundgren movie marathons. An '80s action movie star loosely based on Lundgren appears in season seven's "Last Action Hero (2014)".
When Beckett bumps into Castle at the precinct, and uncomfortably brings up the topic of picking up her belongings from the loft, she mentions she was planning on calling him about said favor, to which he replies "Here I am, roguishly handsome and at your service". In previous episodes, he's always called himself "ruggedly handsome". The character he based on himself in the Nikki Heat books, Jameson Rook, always called himself 'roguishly handsome.'