Disappointed that your copy of Gran Turismo 5 Prologue is collecting dust now that the full title is out? Slip it back in to your PS3, because here's one more secret to find. Mathieu H., on Twitter, has found a secret menu in the budget title that opens up a brand new menu, with images of the Nurburgring Nordschleife, Circuit de la Sarthe, and Tsukuba tracks that supposedly weren't available previously. You can also pull up a secret options menu to toggle availability of various cars and other options, and set up races with certain cars. In other words, this is probably a kiosk mode, used to help set up demo units.
Alas, the code doesn't work in the full game, so Gran Turismo 5 owners will have to be satisfied with, you know, the full game. But then again, maybe there are a few secrets still wandering around that game's menus after all.