引用自 videogames10：

I don't know how many I have at the moment, but I seem to only find them hidden in interior walls and they're very rare. I've yet to find a hidden compartment within an exterior wall. Exterior walls may not contain secret areas, I don't know. I assume you'll have to play the game several times to acquire all of the documents.

Edit: I have nine out of the thirty documents.

I found 2 just laying on the ground though.

I haven't found any lying on the ground I can think of, and none in Maze mode (although I haven't found many hidden walls in maze mode so that may just be bad luck). I got a load at once when I first found out about them and rechecked all 5 zones in story mode (about 6), but only picked up 1 or 2 at most in story mode playthroughs since then, and have about 11 or 12 in all, but it seems a very slow process to get them all.

Maybe we should make a guide in which we include all the notes, because finding them all yourself is way too tedious comparing to this game's content, especially that those notes don't show up in maze mode.