When all of the enemies are dead, look in the holding cells. 3
of them will have a marine, while the 4th one will have Captain Keyes in it. If
you kill all the Covenant when you aren't cloaked, one of the marines will go
"That was amazing sir! You really kicked the s..." Before he drops an s-bomb,
the Captain will interrupt and tell you how to free them all. To do so, go to
the control panel in the back of the room and activate it. All of the cells
will be open. The Captain's cell will be to the right of the panel. You can
either A) go into his cell or B) just wait a few seconds for the cut-scene to
happen once you activate the control panel.
From gamefaqs
