Unarmed Marines

On the Metropolis (Level) it is possible to find a marine without a gun. Fight your way through the level until you get to the area where Cortana says "There's some marines trapped in that building. Concentrate on the Wraiths, they're the biggest threat."

Keep fighting until the doors to the base open. Then carefully reverse park a Warthog or Ghost near the door. Head up the first flight of stairs, then halfway up the second, then head back down to the entrance. If done right, the Scarab should approach. Quickly jump into your vehicle and head to the Scorpion Tank. Get out and kick the driver out of the tank. Save the tank from destruction. if you look at the driver, he will have no gun! This Marine will not take any weapon you offer him.

You can also see an unarmed marine on Halo: Combat Evolved on 343 Guilty Spark. If you accomplish the trick to board the pelican that has just dropped you off, you can travel over the map. On top of the building where you first meet the flood there is a marine you can drop down to. He is completely unarmed.