I always log into the New Posts page. Once I read a thread, it leaves the page. However if someone else replies then it reappears in bold.
If I go to a specific page on a topic, after I read the thread, it will still be on the page, but no longer bold until a new reply occurs. And on specific pages, the threads are listed from newest post to oldest, they don't "disappear" like on the New Posts page. At least that my observation.