There are rumors that lead to that, but there are also rumors that he's seeking out Dr. Halsey or looking for something (which hasn't been revealed to us, however, the director in 343 has revealed that Chief is, indeed, seeking something).
I did some major research, and this was what I came up with.
Whether on his own accord or by the summons of the UNSC (most likely on his own accord, but I don't know XD), chief finds Doctor Halsey hanging out with the elite separatists at sanghelios. He tries to seek answers from her, but later he has no choice but to flee as he is overrun. Whatever he uses to escape, it sustains some damage and he crashlands on one of sanghelioses moons. His armor is severely damaged and breached, thus causing him to discard the now useless chest/back piece. He has to dawn a cloak to protect the rest of his somewhat still functioning gear from the intense heat and the sand storms. Now, it's said that there is forerunner technology on one of the moons, but I cant remember which one (or where I found that info :/ because online sources don't give much info on sanghelioses moons)
EXPLANATION: The planet in the background of the halo 5 trailer looks VERY similar to sanghelios (as depicted in multiple halo sources). That's why I'm most certain that Chief is on one of the moons.
He also may be on installation 00, because it is a desert environment and contains relics from the battle between mendicant bias and offensive bias and is one of the locations where mendicant biases rampant "parts" are still located.
whatever the case, I think that mendicant bias's "mind" may be stored inside the "bird thing". The bird thing MAY be a war sphinx, but it said that the war sphinx was 10 meters high and 20 meters long. Is that too small compared to the trailer?
Another theory, It may not be mendicant bias inside the war sphinx, it may be one of the Didacts themself (or at least their mind(s) programmed)
last theory, the bird thing may or may not be a war sphinx but may be, instead, an ancilla.
anyway, those are my theories XD one of tham HAS to be right... I hope...