Hidden weapons on The Pillar of Autumn
From Halopedia, the Halo wiki
Begin the level at Rally Point Bravo, titled "Keyes". Once you get to the first flight of stairs leading to the final area, turn immediately to your left and go toward the small building. Walk along the left side of the building until the gray material stops; turn right and melee the wall. A supply drop box containing four DMRs and one Shotgun should pop out of the wall and land on the ground. If this doesn't happen, then it might have already come out by itself and disappeared due to a loading point. Alternatively, it could have popped out into the building, rather than out of it.
When the box pops out, it sometimes does so at high speeds, causing it to fall off the nearby cliff. It is also capable of killing the player if its speed is high enough and the player is standing in its way.