Missing Darwin Notebooks Returned to Cambridge University Library

Exactly. But for many people possession of such artifacts is seen as protection against jail. Certain European organized crime figures have used the "return of cultural treasures" as a bargaining chip in plea negotiations. Possession of stolen art, or at least knowledge of where it is hidden, can reduce one's time in prison.

https://www.nytimes.com/2013/11/17/magazine/what-is-the-valu...

"A prison sentence, for instance, might be reduced in some jurisdictions in exchange for a criminal’s help in retrieving a missing Monet. In effect, an unframed canvas, easier to move across borders than its equivalent in cash or drugs, acts as a high-value and extremely pretty bank note."

https://www.theartnewspaper.com/2020/10/13/steal-and-repeat-...

"In other examples, it appears that anticipating the authorities catching up with them is exactly the reason why such prime works are appealing to criminals. “One thing I’m seeing more of is the use of such stolen works as a bargaining chip for [reducing] sentences,” says Robert Read, the head of art and private clients at Hiscox. “This trend was noted about a decade ago, but as sentencing gets more creative in the courtroom, it would appear that criminals are viewing it as more of an opportunity—in this case the more publicity and better known a work, the better.”"

Do you have a reference for that? At least in Poland that's explicitly not the case, and I'm curious how does that kind of legislation distinguish between a forgery and a replica.

This is why for example when the British police run public cold case campaigns they often stress that people might seek to come forward because "allegiances change" -- careful code that hints at leniency while seeking help from former co-inmates who have fallen out with the offender, spouses or abused partners leaving husbands, children escaping the control of malignant parents, etc., or people who have simply survived the now dead offender and are no longer terrified to speak.

I would wager that many stolen objects are quietly destroyed by those who inherit them, rather than confront the difficulties returning them would involve.

Academic institutions are full of people who want to live a quiet life or build a reputation for the right reasons. They are full of people who come and go, most of which wouldn't know what they are looking at or whether it belonged there or not. Libraries and archives are among the few places that will properly inventory and catalogue what they have, and even the Cambridge University Libraries performed several searches to verify an error was not made on their end since it does happen.

It may be unlikely, but I think that it is possible that the process for handling these materials failed somewhere and it took twenty years for someone to come across them. Their desire to drop them anonymously may have been for any number of reasons. Regardless of their reasons, I don't think that doing the right thing should be punished. I could also understand why someone would rather burn something than deal with a slow and possibly misdirected justice system.