Georgia Grainger had only been working at the Charleston Library in Dundee, Scotland for six weeks when she was thrown into a non-fiction mystery of her own.

It all started with a question from a frequent patron who noticed a bizarre trend in the books she was borrowing. They all had a small line drawn underneath page number seven.

After checking the books herself, Grainger realized the patron was right. She found a similar inscription in numerous books, as well as a separate ‘x’ symbol on the ninth page of others. A pattern emerged when she noticed many of the books inscribed were wartime romances.

“I read a lot so I’ve got a wild imagination,” Grainger said. “I started jumping to all sorts of conclusions.”

Her theories about top-secret spy codes and serial killer markings were soon debunked when she brought the mystery to the attention of her manager.

It turns out, these types of inscriptions are very common in public libraries. While many libraries are now equipped with a database to track the books a person has borrowed, patrons are still taking things into their own hands by leaving behind distinct symbols to mark the books they’ve read. Some leave markings on particular page numbers, while others use initials or sign their names beside the date stamp.

“It was a lot less exciting than I’d thought,” Grainger admitted.

Nonetheless, she took to Twitter to tell the story and was met with immediate and widespread reaction from librarians and library-goers around the world. The tweets have garnered over 25, 000 favourites and almost 9, 000 retweets.

For Grainger, the best part has been hearing stories from people around the world about their loved ones who shared similar habits.

While she worries her story could encourage a surge of new library-goers to start marking their own symbols at local libraries, she thinks the impact is a positive one.

“It’s really nice to have this kind of attention on public libraries.”