In a terminal in Halo: CEA we saw a ship crashland on installation 04 … Whose ship it was ? Cause it wasn’t any Covenant ship, given that nobody ever came outside…( Could have been San 'Shyum before The Covenant was assembled) … A mystery we’ll probably never know…
We don’t know yet. There is speculation that this will be explored in Nightfall.
Oh yeah ! Of course almost forgot about Nightfall … Well i hope we’ll get some answers
Oh I see… Well, that is intersting theory, anyway we’ll learn that in Nightfall 
The unknown terminal ship is certainly something I eagerly want to know more about.