Prisoner of War Medallion
From Halopedia, the Halo wiki
The Prisoner of War Medallion is a medal awarded to UNSC personnel who have been captured(POW) 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, John-117 and his team should have received this award, because he was captured by United Rebel Front at Camp New Hope. However, having the Spartans being captured by rebels might have created a bad image for the UNSC. A possible awarding for a Spartan is when John-117 was captured and interrogated by The Covenant at the end of Halo Uprising #1, but this is unlikely because no UNSC soldier or officer was present at all.
Finally, there was another opportunity for John to gain this medal; according to the Halo 3 Diorama , John was captured by the Covenant in order to save a massive bunch of UNSC ground forces.
