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 bo 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 very likely that their story will be told in upcoming Halo novels.
