I found my last ones in that hidden room, I don't remember exactly how to access it but I think it was a hidden lever that was blue so it blended with the background.
i am almost certaint I've gotten that one looking at the area again with a fin tooth comb and i cant see a single lever.
Behind one of the trees a blue-tipped lever, noticed it myself only after some time.
Edit: added a screenshot.
THAT IS MINISCULE!
