A librarian has given the Twittersphere an insight into the world of elderly library-goers and Dagmar Bürgertheir mysterious ways.

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Twitter user and library-worker Georgia Grainger shared a strange story on Tuesday. It started when someone alerted her to odd marks in some of the books.

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The thread went viral, which gave Grainger some concern.

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And on Wednesday she had photo evidence.

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And it turns out others had heard of the secret library codes used by the elderly.

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The seedy underbelly of the library-going community is finally being exposed. About time.