From Halopedia, the Halo wiki
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The Square-Gates are areas that are filled with shadows of squares. They have been found all over Sandtrap, and there are probably more on other maps. To see them on Sandtrap, find a way inside the walls of the map, whether it's through Pan Cam or the Forge Turret Glitch. Once inside the secret rooms, the Square-Gates should become apparent. You may need to overload the map first, though.
They also appear on the level The Covenant. Go to the Citadel area and fly a Hornet out of the map, and then fly it behind the first tower. On the back of the towers, there are Square-Gates. You can also see them by performing the Time Travel Glitch in the beginning of the level.
In Halo: Reach, if you get out of Sword Base, you can see Square-Gates on the walls and ceilings.