The menu on top of the page shows 17 open review tasks for me (number between the mod link and the review link). But when I click on it I get to the review site where is no open task shown. Is this possibly a bug? I noticed it ~14 hours ago for the first time.
These are all review tasks currently pending completion. As you see none available when you click through, that means you've either already reviewed (before you became a moderator) or skipped them.
This gives you an at-a-glance overview of how the system is working: if the number stays static for an extended period of time, you may wish to encourage your fellow moderators and other members of the site to pitch in and help you wrap them up.
You can get a list of pending review tasks using the Stack Exchange Data Explorer: http://data.stackexchange.com/tor/query/188147 - note that this is updated weekly, so some of the tasks listed will have since been completed, and some new ones will be missing.
For more information, see: