The Chinese words on the package are the title of "The Departed", which is written by William Monahan as well. Infernal Affairs (the movie The Departed was based on) was incorrectly called "a Japanese film" during the 79th Academy Awards; Jordan emphasizes "he got it from Hong Kong" here.
When the former caretaker Lee arrives, the Tree air freshener in his van is yellow. When he hurries to leave, the air freshener in his van is now green.
When the camera angle switches between the two main characters while they are in a moving van, the difference in speed of passing scenery is very noticeable.
When the copper first appears at the flat where Mitchell is staying, he clearly introduces himself as 'Detective Sergeant Bailey'. But in the credits he is identified as 'DI Bailey'.