When leaving an agitated Bastion Cole at the nursing home, Cole refers to Hetty as "Gartenzwerg" - a garden gnome.
The venue where Hetty and Mattias meet is the Hollywood Bowl.
Staatssicherheit is actually the Ministerium für Staatssicherheit, which translates as the Ministry for State Security. It was disbanded in 1990.
Callen is driving an Aston Martin Rapide. This car is his most expensive throughout the show, valued at $200,000+ US.
The book Hetty is reading an excerpt from to Branston Cole is The Red Badge of Courage by Stephen Crane