Two 'stolen' Charles Darwin notepads have been returned to the Cambridge University library 22 years after they were last seen.
The books, valued at £10million by a world famous art crime investigator, were returned in a pink bag with a 'Happy Easter' message.
Arthur Brand, who has tracked down artefacts worth millions of pounds, believes that an "aficionado" is behind the 'theft' of the books.
He said the culprit could be a person with good knowledge of the library and its contents or a "trusted" member of the public.
The 53-year-old from the Netherlands added that returning the books suggested they cared about safeguarding them.
According to the BBC, the manuscripts were last seen in November 2000.
Arthur said: "I love the sense of humour with the message. He or she is probably laughing now.
"It seems to be a real aficionado. It's someone with enough sense not to destroy the books.
"In many cases it's ordinary thieves but this was someone who had love for these books."
But Arthur, who in 2019 helped return a stolen Oscar Wilde ring to Oxford University, added that someone with a close connection to the library or a frequent library user was the most likely culprit.
He said: "It's someone who could see an opportunity to take it away.
"I don't think they wanted to sell the books.
"This person probably stored them under their bed and read them every night before going to bed."
Arthur said that police were likely to use fingerprints and CCTV to track down the culprit - but that an arrest was unlikely.
Cambridgeshire Police said today that its investigation remains open and detectives are "following up some lines of inquiry".
Arthur said: "The person probably knew what they were doing. Maybe they wiped off the fingerprints from the books.
"I'm not sure the police will continue with the operation.
"The books are back and the police have more stuff on their minds. I think the case is closed.
"If police arrest someone who returned the books then the next guy thinking about returning something might change their mind.
"This person made a wrong and set it straight."
Arthur, who has nicknamed himself 'Art detective' on Twitter, added that he was surprised to see the books returned.
He said: "After a theft the museum always asks for its return but it normally never happens. It's quite rare for this to happen.
"You always dream of someone doing the right thing and bringing it back.
"Sometimes people feel remorse. Sometimes they discover they just can't sell it.
"They might destroy it to get rid of it.
"Some people realise that they did something wrong and are afraid of Karma."
