We’ve all seen the new trailer by now and duh Chief isn’t dead they wouldn’t outright spoil something like that. The most likely reason is that ONI is covering it up so Locke can hunt him down easier and so ONI can do whatever with him once they have him you can’t petition for someone’s release if you think their dead right. I think the reason they would have done this has something to do with the first Blue team mission when they go to the ONI station. I think Chief finds something out about ONI a currently unknown plan or already committed atrocity because if the covenant is willing to attack and take over the station there has to be something good in there. So he finds it goes missing on Meridian after the Guardian attacks ONI says he’s dead then they go to run him down while he tries to fight off the Guardians along with not really bad guy Locke being ONI’s puppet.
But even though what above is 99% mostly likely the scenario or at least similar to what happens I’m going to speculate a little. Now a spoiler here for you but during one level shown at an event chief falls into a chasm a sees Cortana somehow I didn’t want anymore spoiled so I got out of the video quick after learning that. Maybe after the attack Chief just runs off to try an resurrect her or something and maybe the guardians are the key so he has to wake then up or find them as they wake up and this quest most likely divides his relationship with Blue team because the very first H5 trailer shows him on his own staring at a smaller orange lighted guardian presumably the last one I would guess.