I remember commenting, likely around 2016 about the use of Hypothes.is' counter on their bookmarklet to indicate the number of annotations on a particular page. This is a more passive sort of indicator of presence.

I used to get a little thrill to see people who'd either been there at the same time, or just before me.

It's also reminiscent of Vannevar Bush's "associative trails" in As We May Think (The Atlantic, 1945).