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", are located near or inside large, tower-like Forerunner structures, and in both levels serve to keep the player from exiting the level.

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. In Sandbox, the Eggs fire subtler but more powerful beams directly at escaping players, killing such players almost instantly. 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 overload the boundaries using Trip Mines, and then walk outside of the map. From there, you must use Theater to fast forward to where you exited the map, detach the camera, and fly up to the upper portions of the towers, where the Eggs can be seen. It is also possible to look at the UNSC Aegis Fate using this method; the camera can be moved into its hangar.

To see the eggs on Sandbox, 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.

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.
 * The Sandbox Easter Eggs' beams can be blocked with Forge Objects, rendering the Eggs harmless; Bubble Shields also provide temporary protection.