I'm on Firefox at my office, logged in always. I had not been on the site in a few months, then noticed it when I came back last week. The pop-up window (barely visible for less than half a second) appears to be a little smaller than the browser window size, but more or less in the middle.
Its a Firefox bug. I get it too, only on Firefox. IE and Chrome do not do this for me.
Its like a ghost spam (if thats even a thing lol). Its just a really fast blink, then it goes away. I tried to take a video of it, but my crappy phone doesn't catch it because it happens so fast.