I override the pencil icon that signals the existence of a Category Note in Book details with a less intrusive small red dot.
I would like to have something similar in the book list alongside annotated categories, it can't be done with a column icon because they apply to columns, not to items within a column - e.g. an Author, a Tag, etc.
I use a small blue dot for Category Links, for the sake of completeness it would be good have something similar for Linked items the Book list.
Assuming there's no existing way of doing it, and would require a new feature to be developed:
IMO it would need to be something less intrusive than the default note and external-link icons. For illustrative purposes I am attaching my red and blue dots, and a screen shot of their appearance in Book details.
And, for consistency, a checkbox in Look & feel->Main interface - "Show indication of links and notes on category items in the Book list" should be provided.
BR