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 Wars?

Also, if someone could tell me how to stick an image in here, I'll post up my outline.