An unused Bonus Stage was a level that was originally planned to be added to Sonic the Hedgehog CD. A screenshot of the level can be seen in the 1992 Sega Summer Catalogue.
Not much is known about the level and much of the Japanese in the article is left untranslated. The tilted angle of the level indicates the level probably rotated like the Sonic 1 Special Stage, and 3 reels can be seen meaning there were also slot machine elements. An object with the letter "R" can be seen, possibly a reverse panel like the Sonic 1 Special Stage. There is also a "U" panel which could have sped up the rotation of the maze, similar to what the "Up" panel did in Sonic 1.
A background layer can be seen further back in the screenshot, with a layout which appears as if it could be navigatable. This suggests that there may have been a way to navigate to that portion of the level, though there is no concrete evidence of this.