Castle and Beckett eventually come to the conclusion that Kelly Nieman played a part in the murders, and go on to discuss about it at the loft while Castle is playing with a pen found at her abandoned office. This pen, he finds, holds a flash drive. When he plugs it in, he finds an audio file. It's a song, one that scares the shit out of Beckett. She is visibly shaken.
The song is called "We'll Meet Again" by Vera Lynn. Do we know the significance of this? It cannot simply be Dr. Neiman taunting Beckett. She would not have been so shocked and acted so broken. And since 3XK always troubled Castle more than Beckett, I have a hunch that song is somehow related to her mother.