Click on the the words "latest open thread" in the sidebar (use your browser's search). The tag works if you reach the open thread via discussion, so that the word discussion appears in the URL, but not if you reach it in some other ways, like from going through an individual's recent comments. (I think that there may be some delay in updating these two sources, but they are both up to date as I write this, only two hours after the new open thread. That thread is two hours younger than your comment, perhaps its trigger.)