I've played 2 routes of Kanon (Mai and Ayu). From what I've read, there's an extra route for Sayuri. To accomplish that you need to beat Mai's route, then go back a few days before the end of her route. At some point you are offered a new option. Clicking that should unlock Sayuri's hidden route; although, I'm not certain if it ends with an ero scene.....
For what it's worth, I hope this helps a bit!