“Unplug the Duck” is an expression used when asking a colleague to turn off the lights in his office thereby allowing him to leave work early while others may think he is still at work.
The “duck” part of the expression is a reference to its use as a “decoy”, projecting a false appearance.
Carl: "I'm slipping out of the office early, can you “unplug the duck” for me?
Ralph: "No problem. Enjoy your afternoon on the golf course."