...in the background. Skip to 0:57.
Since it is on "the smallest map in Halo 4" and it appears to be outside the map boundaries, I highly doubt it will be drivable.
Personally I am really dissapointed by the lack of a new Forge map.
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Although I am disappointed that we can't drive the Hornet, it wouldn't make any sense for us to be able to in a small-medium-ish map. I do think it is good that, even though we can't DRIVE it, we know that they still put it in, which is good, in a way.