Why is there always supposedly one unread message with my gmail on my mac?

On the doc on my macbook at the bottom, the mail icon, which I have set up to gmail always has a little red one meaning I have an unread message but I really don't. I have gone through all the mail looking for one that is unread and I can't find it. I don't think it exists. I restarted the computer, and it's still there. Anyone know why, or where this alleged unread message could be? The red number is bothering me.