Hidden Weapons on Regret
From Halopedia, the Halo wiki
Get to the first part where you fight the two Hunters. Walk towards the Gondola, but stop right in front of it. Do not walk on, but instead walk to your right, until you get to the ledge. Now look down off the right ledge. You should see a platform connecting to the giant platform you fought the Hunters on. The wall right beside the platform will hold weapons, stuck half way through the wall, though you can still pick them up. However, it is good to keep in mind that after you reach the hidden cache, you cannot get back up to where you previously were. Additionally, sometimes the weapons will not spawn.