All throughout the game, you'll find the letters "FCK" (on newspapers, boxes, etc.). They stand for FC Kopenhagen, a Danish soccer club some of the programmers seem to like.
In the Rendezvous in Rotterdam mission the truck outside the biker gang club is marked I.O.I, which stands for IO Interactive, the developers of the game, also on the car that 47 used to drive to the restaurant in The Seafood Massacre.
Along with all the missions being at night, they also all have some form of precipitation (either rain or snow).
There exists a melee weapon called "cardboard tube" in the game, simply called "tube" in the game play. The weapon can kill enemies with a single blow, sending them flying in the air in the process. It is a weapon that is retrievable only by using cheat codes.
All the levels take place during the night-time.