Prisoner of War Medallion
From Halopedia, the Halo wiki
The Prisoner of War Medallion is a medal awarded to UNSC soldiers who have been captured in combat by enemies of the UNSC. It is an honorable decoration, though few humans have it. The Covenant do not take prisoners, except the occasional straggler to execute for sport. There are few other enemies of the UNSC that the military contends with, so the medallion is a rare sight.
As of 2552, the Prisoner of War Medallion is the only service medal that has not been awarded to a Spartan, namely the Master Chief.
Technically, Master Chief Petty Officer John-117 and his team should have received this award, because in Halo: Ghosts Of Onyx they are captured by rebels at Camp New Hope.
