(at around 35 mins) there is a nighttime exterior shot showing two sides of a Chicago building. The interior lights on one side spell-out 9/11. The lights on the second side show an American flag. Another building to the left shows some decorative lights in the colors red, white and blue.
Artie, in Dickensen's office, about to rewire something to trace the unknown hacker, says "OK Mr. Knock Knock, I' m gonna open a door and see who you are". However, it is in the next episode that the hacker makes the warehouse electric panel go 'KNOCK KNOCK'.