Inbox indicates unread emails

Hello, My unread email indicator in the left hand column of my email page has been showing that there is an unread email in my Inbox for weeks, but there is no unread email anywhere to be found. I have tracked back through several weeks of emails in my inbox, way past the time when the indicator started telling me there is an unread email, but there is none. Does anyone know why this happens (it's not the first time) and how to fix it? When I filter unread it reads 'This filter will show you all the unread messages in this folder. (Looks like you've read everything already!)'. Many thanks if you can tell me. 
PS I have read the related topic that outlines exactly my problem and done what the replies.