Castle says the Sherlock Holmes Society is meeting at 221b Baker Street, the actual home of Holmes. Untill the 1930s Baker Street did not extend that far, 221b was as fictional as Sherlock. In 1990 the Sherlock Holmes Museum got its address of 239 Baker Street officially renumbered as 221b.
This episode was originally set to be the season finale but then they scheduled two more episodes to air.
This is the second episode to reference Samuel L. Jackson's famous "motherfucking snakes" line from Snakes on a Plane (2006). Esposito first paraphrased it in The Third Man (2010).
The plane Castle and Alexis are travelling on is Oceanic Airlines Flight 57, as well as the TV series "Lost", this also references the Wesley Snipes film "Passenger 57", in which a terrorist plot has to be thwarted by the hero. Oceanic Airlines was also the carrier used in "Executive Decision" which featured a youthful Jon Huertas as a terrorist gunman.
In the end, Beckett watches a computer screen tracking Flight 57's landing at Heathrow. The screen image is a modified satellite image of London. In the close-up, just outside the grey area of London, at about the 9 o'clock position, is a wide grey rectangle over a smaller rectangle. That is Heathrow Airport.