Google is testing a new AdSense ad format with select publishers, commonly known as sticky ads. Google is calling them anchor/overlay ads and vignette ads.
The Anchor/overlay ads are ads that are mobile specific and they stick to the edge of the user's screen and are easy for the user to dismiss. The vignette ads are also mobile only and are full screen ads that appear between the page loads on your site and can also be dismissed by your users.
You may be able to see them on the page-level ads section of your AdSense console.
Forum discussion at WebmasterWorld.