Halo: Reach Easter Egg - Secret Weapon Cache (NOT an egg)

At the end of the second to last level (next to the lone wolf bonus level), Emile should be on a MACH gun and you should be clearing the platform for Commander Keyes. On the warehouse with useful guns to the left, go to the back of it and keep meleeing next to the window. If you hit it right, a weapon cache will pop out with useful DMR guns and shotgun ammo.