Hidden sniper rifles
From Halopedia, the Halo wiki
The first Sniper Rifle/weapon stash is located directly after the Hunter battle. When you walk into the clearing where the obvious highway used to be look to your right and there should be two boxes. Jump on them. To your right should be the stash. You should know it when you see it because several dead Jackals, Grunts and Humans will be lying there (signs of struggle) as well as pistols and rifles. Also, this is the area where the Pelican picks up Sergeant Johnson (and drops off new Marines). The end of the next alley, nicknamed "Sniper Alley", has a hidden Sniper Rifle and ammo.