Wait, the common rules of this kind of threads are to guess tracks are accessed in the game by normal means.
If your going to include hidden tracks that can only be accessed by breaking codes and use other kind of software, then I don't think that counts.
Although I do remember something like that when GT PSP came out.
The intro features the Nurburgring GP pits garage, when the Corvette and GT-R take off.
If that's what you are talking about...well, it's a location in the game, but not a drivable variation.