Command shuttle
From Halopedia, the Halo wiki
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Not much is known about the Command Shuttle except that after all surving marines were brought there as said by Cortana or a marine in the level Halo
Speculation
This could be were Keyes is were he is transmitting his message to the Cheif during the fourth level. He can't be on the Pillar of Autumn for it was crashed. However he could be on a pelican.
