You do have a point in the swirled up dust, but the problem is, that makes it even more likely to be an aerial vehicle, those kick up ALOT of dust when landing/taking off, you can even see its shadow, distinctly pelican shaped. I did read on Halopedia, that in Halo 3 an assault pelican was going to be available, why not in Halo ...

I wouldnt say those are scorpions or hogs, note the fact that you dont account for the entire outline, what you call protection is the rear stablizers of some aerial vehicle, and how do you explain the four tubelike projections under it?

Well, I took the pciture from thunders sig, and messed with some settings on brightness, contrast, and certain colors of pixel remover, and we are left with what looks like an Assault Pelican Gunship, or a Skyhawk Jumpjet in my opinion. Though you never saw a skyhawk in the games, and it wasnt very descriptive in the book with the exception of 4 ...