This is why the number one priority of every library with rare books/manuscript/maps should be to get them digitized and distributed as widely as possible. These old works are out of copyright anyway, so there should not be legal issues to doing that.
Basically, anything that is stored in a single place is vulnerable.
If we assume there are people in possession of these notebooks, what could incentivize them to return it? Is there any value in holding onto the originals?
Something that has happened in the past is an employee stole a valuable book(s) and then died and their children inherited it without knowing what it was. The public appeal reached the kids who then returned the books.
More like Christie's, ebay for the rich. They have been caught selling stolen artefacts so many times they now try to "investigate" if the seller has the right to sell.
Basically, anything that is stored in a single place is vulnerable.