Just, based on where the Pelican is. Was Chief trying to establish a base, or perhaps working with other UNSC assets in an attempt to utilise ‘weapon containment’ chip to seize and eliminate Cortana?
Perhaps, we’ll have an epilogue where Blue Team, Arbiter & Chief are together before the ring is broken and either Cortana destroys that section of the ring or Anders does to stop Cortana from ambushing the remaining UNSC forces. This would explain why Chief is unaware of the current state of affairs, why he’s alone and additionally why his butt is floating in space AGAIN!
I think its a fair conclusion that Chief was on the ring during the battle. It’d explain why he’s adrift in space incapable of moving, similar to Halo: 4 when he nuked the Composer. As for the details of the battle or what he was doing, we don’t know for sure. All we really know is that he was fighting on the ring, and some major event happened, which resulted in the damaging of Zeta Halo(presumably) and Chief ending up in space. I have a few personal guesses:
-Guardian collided with the ring , resulting in the damage dealt to it
-Potential “nukage” on or over the level as the Composer was, possibly to eliminate a coming threat that was on the ring(Flood, perhaps?)
-UNSC Infinity collided with the ring. Explosion of the Pillar of Autumn was capable of damaging Installation 04(Alpha Halo), therefore it could make sense that the Infinity’s reactors could destroy Installation 07(Zeta Halo) in some capacity
-Some Forerunner-based weapon capable of causing that damage being activated/used
-This one just clicked onto my mind, and its wild, but eh, it could happen. A natural defense system on the ring, similar to that of what Anders did on Anders’ Halo, lifting pieces of the ring out. The only thing I can really justify it with is the game’s supposed cover art, which shows parts of a ring being flung out in a similar fashion.
Nonetheless, those are all guesses. The idea of a sort of prologue before, though, would make some sort of sense to get us up to date on what happened to lead us where Chief is right now. Heck, maybe we’d have to find those pieces from terminals or something to piece the lore together ourselves.