But I think he wore the cloak, because he was running from the UNSC or ONI or whoever.

Think about it.

Don't you think that Master Chief would have some sort of detail assigned to him is kind of strange?

I imagine there has been some time between Halo 4 and Halo 5.

His armor looks much more weathered, and how about that Crack in his Visor?

Possibly signs of a struggle.

And that planet is definitely not Earth.

My theory is that ONI tried to ship Master Chief away because of his experiences would disagree with their motives (see: the Kilo-Five books)

Master Chief probably didn't like that, and I imagine that he went off in search of Forerunner Artifacts, possibly trying to find a way to bring Cortana back.

As for the planet, it seemed to be a battle stricken planet, possibly a previously glassed planet?

It would explain all of the sand.

Let me know your guys' theories below.