During the "Scorched Earth" campaign mission in Berlin, hidden in an alcove immediately to the left after walking into the Trivo Buchhandlung ("Trivo Bookstore" in German) are a shelf and display of books adorned with the names of numerous developers who worked on the game. The most notable kind of book in the display however features the name Guy Beahm and a black and red circle with a pair of sunglasses and a moustache. Beahm is better known by his online alias Dr. DisRespect since 2010, a character known for his over-the-top personality and appearance, including the same sunglasses and moustache that appear on the book cover. Beahm was hired by Sledgehammer Games as a Community Manager in 2011, and the following year accepted a Level Designer position in the studio where he designed multiplayer maps for Modern Warfare 3 and Call of Duty: Advanced Warfare. Beahm left Sledgehammer in 2015; this Easter egg went largely unnoticed until 2022.
Though the enemy weapons have tac-lights, once the player acquires one of these guns, the flashlight disappears. This is due to the fact that the lights are attached to the enemies themselves; not the guns.
References to two cut Spec Ops missions can be found in the game's files. The first one is Defend Ferris, and judging by the lobby image associated with it, the player would presumably have had to defend a ferris wheel. The second is named Assault Mine, and its lobby image shares some similarities with the Spec Ops mission "Fire Mission", but there is otherwise nothing known about it.
A day prior to the game's European launch, two masked men in south Paris, France collided with a delivery truck and stole a shipment of the game. The delivery was estimated to include about 6,000 copies of the game with a total value of over $500,000. This is considered to be one of the biggest video game heists in history.
Near the east corner of the map Village is a hatch with the letters "4 8 15 16 23 42" written on it. This is a reference to the TV show Lost, where these numbers were used frequently.
On the hammer of the Model 1887, the words "Hasta La Vista" can be found. This is a reference to the film Terminator 2: Judgement Day, which made the phrase famous.
It's believed that there was a game mode called Grinder that never made it into the final game. This is speculated because of a number of unused sound files found in the game's code.
There were 3 planned killstreaks that were removed in the final game. 2 are from previous Call of Duty titles. The killstreaks are Harrier Air Strike (From Modern Warfare 2), Emergency Airdrop (From Modern Warfare 2) and Dragon Fly Drone (Reused in Black Ops 2). Voice files of these can still be found in the games files.
There are various weapons that were cut from the final game but can still be found in the game files, these include: FAMAS, M1014, MK12 SPR, MP5K, Vector, AUG, F2000, W1200, WA2000, STG44 G3, M93 Raffica, Remington 870, Lee-enfield, MG4, TAR-21 and Scar-H. All of these weapons (Besides the MK12 SPR) are from previous Call of Duty titles and were likely removed to make way for new weapons.
On the multiplayer map 'Sanctuary', when the player lays next to various gravestones in the cemetery area of the map, ghost like voices can be heard which are linked to the campaign of the game where a mother, father and their daughter all die from a terrorist attack.
While searching through the games code you can find an image of a remote sentry in the style of a PERK used in previous MW games. Making it seem like remote sentry was originally planned to be a perk in MW3