Bulletin board
From Halopedia, the Halo wiki
On the first and last of the Halo: Combat Evolved levels, a bulletin board can be found just outside the bridge of the Pillar of Autumn. Some of the messages look plausible, some are just silly, others are hidden Easter Eggs.
One says: "Missing: Calico Cat: Answers to Jonesy" This is a reference to Alien and Aliens, two movies that Halo borrows from many times, to include the banter of a Marine soldier. Jonesy was a calico cat in the series, the only survivor of the first movie, along with Ripley.
There is a picture of Earth from the Moon with the caption "Fight for her" on it. This is likely propaganda within the UNSC. It also shows Africa, and may be a reference or foreshadowing to where the Ark's portal is hidden.
There is also a note reading:
For Sale:
Pentium XI 33 GF
Chicago Office
Colony Ship
*3 Speed Manual
*Transmission
Hyde Park Condo
Integrity
This is a reference to Bungie being bought by Microsoft. It mentions the location of Bungie's old office, as well as referencing a stage in Marathon, entitled Colony Ship for Sale: Cheap.
There are other things too. Some you can read, others you cannot. Also you can see the words "Marines Keep it Clean" and "Scooters are for Squirts, No Scooters on Deck".
Another says "Attention Marines callbacks for South Pacific". The bottom of the poster is cut off by the keyboard.
There is actually more than one bulletin board on the Pillar of Autumn. On the mission The Maw there is a shattered board on level one right through the cafeteria were you find two medpacs and a empty shotgun, it should be on the wall. Also the Custom Map Team remade the board. Referencing team members being sold for "butt secks"
