Deciphering "wee old lady" library book code

Georgia Grainger, a Scottish librarian, began a fascinating Twitter thread earlier this week:

So there was a MYSTERY at the library today.

A wee old women came in and said "I've a question. Why does page 7 in all the books I take out have the 7 underlined in pen? It seems odd."
"What?" I say, thinking she might be a bit off her rocker. She showed me, and they did.

— Georgia | Saoirse (@green_grainger) April 3, 2018

I asked if she was doing it, she said she wasnt and showed me the new book she was getting out that she hadnt even had yet. It also had the 7 underlined! "I don't know, maybe someone really likes page 7?" I said, assuming of course that there is a serial killer in the library.

— Georgia | Saoirse (@green_grainger) April 3, 2018

I checked some other books. Most didn't have it, but a lot in this genre did - they're "wee old women" books (romances set in wartime Britain etc). Lots of underlined 7s. The woman who pointed it out shrugged and went on her way, "just thought you should know".

— Georgia | Saoirse (@green_grainger) April 3, 2018

My manager came back from doing arts and crafts with some of the kids and I decide to tell her about the serial killer in the library.

And that’s how I found out that a lot of our elderly clientele have secret codes to mark which books they’ve read before.

— Georgia | Saoirse (@green_grainger) April 3, 2018

Just had another victim of the page 7 vandal returned!!!
(Now checking every book that looks like it might be their taste...) pic.twitter.com/SGHST4kcOv

— Georgia | Saoirse (@green_grainger) April 4, 2018

Turns out from the ensuing comments that this is a rather common practice. A dot on page 50, a circle around the number 40 on page 40, a little "P" on the title page, other librarians and readers started to chime in with the codes they have seen. It seems to be mostly elderly patrons and those reading from sections like Romance, where one can burn through a lot of books with self-similar subject-matter. See the entire Twitter thread here.