SPEEPSHighway | Sonic Adventure: Final Egg's 4th Act @SPEEPSHighway | Uploaded 8 years ago | Updated 3 hours ago
Final Egg has a weird 4th part that has some pretty interesting stuff in it.

There's a lot of stuff around 0,0,0, but as Sonic you spawn really far away from everything.

Seeing as how the autodemo version has a copy of the Egg Viper's cutscene map in Final Egg 4 and Final Egg 3 has one part of that map that's blocked off in the final, i'd say you were originally planned to go to that area at the end of Final Egg instead of ending the level immediately after the last ring trail.