Well, I wouldn't say it's one of the Halo's, because when 343 showed all 7 of them up on a holo-pad thingy, none of them seemed special, except one, which had a a line coming off of it, and going to a box with blinking red letters in it, which I surmised to be the one destroyed in Halo: CE. I think it's pretty obvious that the structure is The Ark. You know- remotely detonates the Rings. The fact that it has similar markings doesn't make it a Halo, it just means that it was built by the Forerunners.