A random floating ad is always on top 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?
You've got some kind of adware. Check your running apps and stop any apps that are unfamiliar-looking. Be sure to checked cached processes as well.