Da Vinci Code objects

The Da Vinci Code Objects are untextured objects that appear in some of Halo 3 's levels. These objects end up using a default texture: a grid of letters and numbers. The name was likely inspired from the default texture's slight visual similarity to the "cryptex" featured in Dan Brown's The Da Vinci Code.

Easter Eggs
  On Sandtrap and Sandbox, spherical untextured objects have been found. These objects, dubbed "easter eggs" and also known as "Da Vinci Eggs", are located near or inside large, tower-like Forerunner structures, and in both levels serve to keep the player from exiting the level.

Sandtrap
In Sandtrap, the Eggs fire orange beams into the sand whenever a player tries to exit the map; these beams activate Brute Landmines that are buried along the level's perimeter. The beams themselves are harmless, unlike the beams on Sandbox. In both cases, the Eggs' kills are attributed to The Guardians.

To get a closer look at the Eggs in Sandtrap, you'll need to use Pan Cam to exit the map in Theater and fly to any of the towers. The eggs will be near the top-right of the towers.

Sandbox
In Sandbox, the Eggs fire subtler but more powerful beams directly at escaping players, killing such players almost instantly. The beams on Sandbox are also much more accurate, as they were meant to target specific players. However, unlike the beams on Sandtrap, Sandbox's beams can be blocked by Bubble Shields and most Forge Objects. These beams are very similar to the projectiles fired by a Gauss Warthog.

To see the eggs, two players must cooperate. One player (Person A) must, while in Edit Mode, exit the map from the Sky Bubble by spawning a Teleporter outside of the map. Person A must then fly to a tower (but watch out for kill barriers) and spawn a turret against the light in the tower. A second player (Person B) must then man the turret and have Person A pick the turret up. Person A should drag Person B up the wall while B is still against it. Towards the middle of the light, the Easter Egg can be seen.

Da Vinci Cube
 On Heretic, an untextured cube is hidden inside each of the lifts at the center of the map. To reach one of these "Da Vinci Cubes", a player must place a Machine Gun Turret against the top of the lift. They should then proceed to use the Forge Turret Glitch to enter the small compartment that holds the cube.

The Da Vinci Cubes power their respective lifts; overloading the map will cause the cubes to vanish, disabling the accompanying lifts. There are only three known Da Vinci Cubes on Heretic, and their purpose is unknown -- lifts on other maps, like Sandtrap and Valhalla, do not appear to require such cubes. It is possible, however, that Da Vinci Cubes exist on those maps and simply haven't been found. Interestingly, the Cubes are noticeably smaller than the Easter Eggs on Sandtrap and Sandbox.

Da Vinci Towers
 In the Halo 3 Campaign level The Covenant, the Time Travel Glitch can be used to reveal three Da Vinci Towers hidden outside of the level boundaries. The towers do not appear in normal gameplay, but seem to be glitched or altered copies of the shield-generating towers that the level's objectives center around. Most of the towers' surfaces are untextured, and the non-solid towers don't appear to serve any purpose.

It is possible that Bungie created two versions or layers of the normal shield-generating towers. The outer layer is immutable, but the inner layer (the "da Vinci" layer) will vanish if the shield generators in the towers are disabled. It is entirely possible that the Da Vinci Towers are projecting the beams that power the shields, which would explain why disabling the shields removes the towers.

Trivia

 * The default texture appears to be a UV-unwrapped image (used to help artists paint textures onto 3D models). The image consists of a grid, with each tile containing a letter of the alphabet.
 * More untextured objects can be seen in the Halo 3 ViDoc "Et Tu Brute?".
 * The reason why Bungie may have left these objects untextured may be that they did not expect players to be able to reach these objects as they are usually well hidden and inaccessible without certain glitches.