Gray Team
From Halopedia, the Halo wiki
Gray Team is the mysterious designation for the team of three Spartan-IIs that were said to be "on battlefields too distant to be easily recalled" in Halo: The Fall of Reach. The team was further identified to have been "on a mission far outside the confines of UNSC space, now missing for over a year"
Nothing is known about the three Spartans. It is possible that their story will be told in upcoming Halo novels, or could possibly be part of Spartan Group Omega in Halo Wars. There is speculation whether Gray Team is a ambiguous term given to the collective of the three Spartan-IIs, so as they could be grouped together because of their similiar mission status, or if they are actually deployed on a mission together. The importance of the missions these Spartans would be on would be of the highest possible priority, having equal or more importance than the Spartan's mission to board a covenant ship and capture a Prophet.