Seems that it was just a Ghost, thanks to Thescourse for pointing that out.
In this trailer there are a multitude of images displayed within a few seconds of each other, one of these, which appears at 0:40/0:41, features an object of unknown purpose or origin, and what happens just before that object appears.
Here you can see the mystery object, and here you can see what I think happens just before the previous picture. It seems like the whole cliff side vibrates and Master Chief looks over the edge to see this falling judging from the few milliseconds of screen time it got. Either way it seems to be flying, as it is in the clouds.
Any ideas on what this is? I don’t recognize both the shape and the patterns. Also, just after this clip there is another short clip that features something that looks human falling on the Chief, feel free to speculate on that too. Note that these clips are nearly impossible to see unless you pause at just the right moment.