I have seen this when I created a new sub-category. There is always this “About [Category]” topic, which is kind of semi-visible.
I have the main category in the sidebar. After creating the subcategory, the sidebar showed “1 new”. I clicked on it, showed the main category list and – nothing in it. As I realised what I have just done, I went into the subcategory, clicked on the “about” topic and the notification went off.
I don’t think this is your case but maybe it will help.