Walking Carpet Disposal | Halo 2: Anniversary 

The mining platform that was harvesting the atmosphere was attached to a rig of sorts. One would argue why they would even bother since that rig is likely just floating lol. Also the funny part about rescuing the Marines in this mission; if you are lucky, you'll hear "good to see you, Chief!". Meaning that they basically just assigned you Chief's biped but swapped the model out for Arbiter to allow for friendly interactions when really all they had to do was add the UNSC to team '2' or 'the Covenant' for this mission. It's not the first time the UNSC has had to align with a foe, since 2492 I think, they'd been at war with the outer colonies. You can imagine how quickly that changed when the Covenant arrived in 2526 lol. Johnson likely saw the Elites cutting down Brutes and leaving his men alone and put 2 and 2 together; the Brutes and Elites are no longer friends. Enemy of my enemy, and all that. Halo 2 is incomplete, despite how many missions there are. Because it was rushed in development, they had to basically add in new material to give it a good stopping point. Halo 3 is essentially Halo 2 pt 2, with ODST being a filler in between Halo 2 (after Metropolis) and Halo 3. They actually even changed the outcome from what they were intending.