The Dunder Code

In the cold open of Season 9's episode Customer Loyalty Dwight is cleaning out some "dead files" and discovers a letter from Robert Dunder which describes a "valuable item" that he has hidden somewhere on the property. Listed below are the clues that lead Dwight and the other office workers on a quest to find this item.

Clues

 * Invisible Ink on Letter- Dwight, using the clue "light bulb" from the letter, holds the letter up against a light bulb and a message is revealed.
 * Higher Than Numbers Go- Guessing this message refers to the ceiling above the accounting desk clump, Dwight jumps onto Angela's desk and lifts the drop ceiling tiles. Another clue is discovered there.
 * An X- A key attached to a tag with an X on it. Thanks to help from Oscar, this leads Dwight to the annex where, in Clark's desk, he finds a hidden compartment with a scroll inside. On the scroll there is a two word message in another language.
 * Sedes Introiti- Dwight interprets this to mean "seat of entrance" and he moves the search to the couch by the door to the office. After ripping up several cushions Dwight finds 5 playing cards.
 * A Flush- Kevin recognizes the cards as being a flush, which is a hand in poker. In the next shot Dwight is seen bringing out a plastic bag from the men's restroom. From the bag he removes the next clue.
 * Miniature Forklift- Everyone unanimously agrees the quest must move to the warehouse and in the next shot we see the office workers have torn through many boxes of paper and possible other things. After some amount of time everyone decides to give up.

"The Holy Grail"
Exasperated, Dwight says, "Forget it." Then the camera moves to show us Glenn seated at a desk and dipping a donut into a golden chalice.

Flashbacks and Phone Calls
The quest is intercut with flashbacks, complete with his old hairstyle, of Jim creating The Dunder Code as well as phone calls between Jim and Pam. From the phone calls we learn that Jim stayed late for a month to create the quest but does not recall whether or not it actually leads to a "holy grail".