On his way to the ransom drop, Castle uses the phrase "five by five" to acknowledge an order Beckett gives him. This was the favorite phrase of Faith on Buffy the Vampire Slayer (1997); Nathan Fillion appeared in the final five episodes of that series in 2003.
The year before guest-starring in this episode, Judy Reyes was in a movie that was also titled "Little Girl Lost."
This is the first episode where Castle brings Beckett her coffee, "Skim Latte, two pumps of sugar-free vanilla syrup".
In one scene, Alexis mentions a warning Beckett gave Castle, which is implied to have happened off-screen, about taking pictures of crime scenes with his phone. This likely stems from Beckett's outrage at him for doing exactly that in "Hell Hath No Fury (2009)".
The character Juan Restrepo (played by Francis Capra) wears overalls embroidered with "Terlesky Auto Detailing". The episode was directed by John Terlesky.