Library books have a wonderfully lived-in quality, with evidence of past readers spread throughout their worn spines, stained or bent pages, and underlined text. But for one library in Dundee, Scotland, this charming wear and tear briefly took on a somewhat sinister presentation: Many books were coming back with page seven mysteriously underlined.

The ritualistic defacing was noticed by a library assistant named Georgia, who took to Twitter to unspool a short narrative.

The “wee old woman” was about to check out a new book—sure enough, it also had page seven underlined. Initially, Georgia joked that a numbers-obsessed serial killer might be among the patrons. But after asking a supervisor, she discovered that the truth was significantly less sinister. Most of the books with the marking are historical romances that are favored by older readers. According to the supervisor, this “wee old woman” genre is often marked up so readers will know if they’ve read a particular book before. One patron used the page seven underline; others put a letter on another page, or a star on the last page.

According to Georgia, this book graffiti isn’t actually necessary. In her library system, she can scan books to see if a particular member has taken the book out before. But in Dundee, it appears that patrons prefer to keep track of their reading history the analog way.