Unarmed Marine
From Halopedia, the Halo wiki
This is a glitch where the very last Marine in all Pelicans have no weapon but their hands are in the shape of a Rifle. When they get out of the pelican the rifle appears in their hands. Also if you manage to save the Scorpion Tank in Metropolis before it gets destroyed by the scarab, the marine that is riding the tank is unarmed.
