In the following image, you can see a floating ad and it looks like an average ad by Google. Problem is, it's always on top. In the image, you can see it over my Settings app.
(Click image to enlarge)
I did not install any new apps on to my device (OnePlus One running CM12s - Android 5.0.2), and the single ad keeps popping up randomly in different positions on the screen. (Since today morning.)
Following is a screenshot of the webpage that opens up, when I select 'opt out'.
From where did this come from, and how do I get rid of it?