There's more!
blazest actually tipped me off (indirectly) about this with a Lost Colony video, though at the time I didn't know what it was. So of course I went to look and sure enough, it really is a Hornet-3 behind the back wall of the starting room, for what appears to be no reason.
From the point shown in this picture, I can't complete the level without restarting, but I can choose to have my method of death be Time Over if I wanted. Basically Sonic is stuck against the wall in delayed-somersault mode, as seen in the first Sky Rail segment of
SA2GT2. However, the kill plane seems to be a bit below the actual surface of the water here, such that Sonic can be half-submerged and still not succumb to it.
And some notable shots with the Sonic-in-other-levels code:
This is simply a clean view of Security Hall from the outside, while Sonic is stuck in a safe (well, not really "stuck", as jumping from inside a safe will automatically open it, but placed there intentionally to get the camera out).
And Sonic can get to the end of Prison Lane! Except not (or at least not without moon jump). If I walk any closer, the final gate appears instantly in its closed position, because one of the three remaining enemies that are behind that door (too far away to hit with Magic Hands projectiles) will come into draw distance.? Too bad...
Another close call. If only this small ledge were big enough to start a superbounce from, it's likely that Sonic could complete Meteor Herd 3 using only his standard abilities. As it is, he'll just have to settle for being 1 wall and about 15 feet away.
Once a way to explore the underworld was revealed, someone managed to find one of these tanks out in the middle of nowhere. It turns out there are several of them, just above the kill plane, and just beyond the turnaround barrier so that Knuckles can't reach them. Sonic, however, doesn't have a turnaround barrier at the "edge" of the level and can therefore get up close...but for some reason, the tanks around the perimeter of the void aren't even solid! What a waste.
