From Halopedia, the Halo wiki
They went to exercise "up north" on Onyx where they disappeared after having reported sightings of "floating eyes". Fellow Spartan-IIIs thought X-ray had been killed by drill instructors who then covered it up. ONI came to investigate, called the area "Zone 67", and forbade access to it. It was discovered years later by Kurt-051, Catherine Halsey, Franklin Mendez, along with the surviving Spartan-IIs and IIIs, that they had encountered the Onyx Sentinels and were presumably killed by them.
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