Pop-Up "Norton" Ad

Even though I have "Special Offer Notification" set to "off", a little while ago I had a pop-up ad appear for a game "from the creators of Norton Internet Security" or something like that.  (Unfortunately, I closed it before paying attention to exactly what it said.)  I'm running NIS 19.9.0.9.  After the pop-up appeared, I ran scan with both NIS and Malwarebytes--both came back clean.

 

Any idea what this was and why it happened?

 

Thanks.