Oooh, quite a find there. I assume that's in the final, right? If so, might be worth looking through some of the Chaotix protos that were part of drx's mass proto release when you're done; see if there's any leftovers in those that were cleaned up before the final release.
Looking at it, the first thing that comes to my mind is the S3&K ending sequence. Isn't that, like, the only other time we see the plane flying to the left of the screen? In fact, come to think of it, doesn't a larger piece of artwork of Sonic & Tails on the Tornado appear in Chaotix's good ending? Could this at one point have been intended as part of an ending sequence? Whatever it's for, I see the Chaotix art team actually bothered to fix the "SONIC" on the side - S3&K just mirrored the plane for the ending sequence since it's seen flying right in all other instances, so the letters were mirror-image in that game.
Looking forward to any further discoveries! = )
This post has been edited by McAleeCh: 02 March 2010 - 12:29 PM