In Halo 3, there is what looks like a purse on the level "The Ark". If you follow your fellow marines to where you first meet the Covenant, go right outside the cave and climb up the rocks, this is tricky and you might have to run up to the rocks and jump on them to climb. If you keep climbing until you can't press on any farther, you should be able to see a structure's ledge. If you use the sniper rifle's scope, you may be able to see what looks like a purse or satchel. You will have to scan the area if you want to see it.