Sonic Unleashed has a few unused levels, level designs, and various other things which can be accessed through gameplay or by examining the contents of the disc. This page notes some of these things.
After the Goal Ring in Jungle Joyride's main Night stage, there is a sealed door. By using the wall-walking glitch, it is possible to cross through this door. There is a large, unused, empty chamber within. If accessed from the ghost mission instead of the main level, the door will be open, and there will be objects within. There is a tree which can be knocked down and used as a bridge, floating platforms, and switches that raise the water level. The platforms and the kill zone raise, but the room's unique water level puzzle is clearly unfinished- the water graphics remain in place. At the top of the room, there is a door. Beyond the door is an ornately detailed chamber containing the original Goal Ring.
This area is used, in modified form, in the Adabat and Empire City Adventure Pack DLC.
After the Goal Ring in Arid Sands' main Night stage, there is a sealed door. By jumping onto it from a well-positioned water barrel and using the uppercut, followed by the midair --/-- combo, you can fall into an unused canyon area. It's empty, but you can open the sealed door from the other side. Bring water barrels to the bottom of the chasm, stack them, and do and uppercut-grab to go on. There is a huge chunk of level which is empty. Go on through it. There are, among other things, two pillars you can knock down, which don't appear elsewhere. Eventually, you will reach a pit, which is uncrossable without objects to bridge the gap. Beyond it is a fairly large stretch of level, leading up to the Gaia Temple.
This area is used, in modified form, in the Apotos and Shamar Adventure Pack DLC.
Examining the contents of the game disc reveals what some of the regions' prototypical names. Some of them had the names of the real world locations they were based on.
--- For comparison, a list of the areas and the locations they were most likely based on would be:
Several files on the game disc are never used.
"E3title.ar" and "E3Title.arl" are present on the game disc.
In the opening cutscene and the one where Dark Gaia rises towards the end of the game, inside Eggman's Eggpod there's a Dreamcast and an Eggman game of some sorts, which seems to be a parody of Sonic Adventure. There is also a NiGHTS game using art from NiGHTS: Journey of Dreams.
In the EggmanLand World Hub, the robots there are named after Previous Sega consoles and games.
In Spagonia, at night, in the first alleyway to the right, you can hear someone singing the Sonic Unleashed theme, which sounds eerily similar to what Sonic whistled in the teaser.
Dark Gaia minions: