SPIRIT

Explanations vary. The Great Masks were hidden by the Turaga to act as means of testing and training the prophesied Toa Mata for when they should arrive. Potentially this was also done with their Noble Masks, but it does appear that Makuta's Rahi also played a part, either by guarding the masks or outright stealing them. The storyline does not go into a whole lot of detail of the Toa Mata's quest for the Noble masks, but we can probably assume that the Turaga did not willingly part...

(Ugh, there's a limit? Gaaaaaaaaaaah! It doesn't even save my original text!)

So yeah, the Turaga probably had their collection of Noble masks stolen by Makuta's Rahi, unless they were dumb enough to hide them along with the Great Masks.

As for the Kanohi Nuva, those were forged by Artakha and his Matoran crafters after the Toa Mata got Nuva-fied, and they were then teleported randomly across Mata Nui. The weird part about that is that in Tales of t...

(Like, it doesn't even tell you what the text limit is! What gives?!)

The weird part about that is that in Tales of the Masks, the Turaga have inexplicable knowledge about where the Kanohi Nuva are, but I guess Greg forgot he wrote that since that was a few years before mentions of Artakha started popping up in the story.

In summary: the Great and Noble Masks were scattered by the Turaga and/or Makuta's Rahi; the Nuva Masks were scattered by Artakha.