Just a guess here, but it's probably some sort of malware. ;-)

How long has it been appearing? I would try to reset back to a time before it appeared. I once had a computer that I loaded a "steam powered" game on and even after I deleted the game from programs it kept popping up phantom screens at start up - that would disappear after a fraction of a second. So I knew they leave some start up junk. I never could get rid of it, even resetting Windows to prior to the games installation. So a lot of those companies are not anyone's friend.

I would tend to think US government spyware would be so good it wouldn't need a click on a pop up and advertise its presence. Maybe some Russian spyware? If a Russian is the only other person on Earth whose known to have seen the pop-up, it may be a gift from Putin or at least Russian in origin.

Good luck.

I still regret updating from XP to 7 and 8 on most of my machines. And Windows 10 is *literally* just window dressing you can roll back completely (within 30 days or so) that was designed to breach privacy even further and sends *all* your computer program, folder and file activity to MS so that can sell it to the government and any other bidders.

Speaking of Microsoft, and on a bit different topic, has anyone seen Bill Gates lately? He's aged visibly about 30 years in 10 - between 2000 and 2011. I suspect he's been exposed to some sort of DNA damaging radiation -maybe by flying around too much and experiencing too many x-ray bursts. But no one around him probably even has warned him about that. The rich who fly a lot age far faster than those who don't. One can have all the money in the world, but without key information one gets screwed over somehow. It's the story of the malevolent genie who packs a poison pill inside your wish.