Phil, here's a new one. I sometimes surf the site on my phone (Android). I have an icon that takes me to a list of threads with unread posts, just like the bookmark on my computer. That list of threads is usually between one and three pages long, depending on how long it has been since I last looked at the site. Last night on my phone, that icon took me a different page, I think labeled "recent activity". That list of threads was very long, maybe 15 pages, and was notable for its lack of a "Mark as Read" button for each thread.
I have not changed anything on my phone. Has something changed at the site? By contrast, there was no difference this morning when I used my computer to check the site - I was directed to a 3-page list of unread posts.