Homages
Throughout the film, Scorsese used Xs mostly shown in the background to mark characters for death; examples include shots of Costigan walking through the airport while talking to Sgt. Dignam (Mark Wahlberg), Queenan falling to his death (on the building glass windows as Queenan falls to ground), and Sullivan returning to his apartment at the end of the movie (on the hallway floors). This is a homage to Howard Hawks' classic 1932 film Scarface.
Read more about this topic: The Departed