The web is dark and full of terrors

Reposting this question, also asked on Tomshardware.  Does Bitdefender popup an add banner like this?

I was doing something away from my computer, which I had just started up. I looked over, and on the screen in the lower right hand corner (I would say about 2/5 of the vertical screen size) was a banner that had in large letters "The web is dark and full of terrors". It was a transparent banner, in that it showed my background behind it and the letters either popped out or in from the background (with borders, the letters were also transparent). There was other smaller writing above and below the main large letters, but I couldn't read them from across the room. I raced over to read it more fully, but it disappeared once I reached the computer.

I'm trying to find out what it was - I thought maybe it could be an add to renew my Anti-virus? I have Bitdefender Total Security 2016 and Malwarebytes installed. Or maybe it was a Steam add? I have no idea.

Or someone has hacked my computer and is playing with me. I'm curious whether anyone else has ever seen this. I tried googling it but get a lot of Game of Thrones pages.

I should note that I did not have a browser open at the time.