I'll keep an eye out for it and capture a screenshot for you. I've been trying for a little while to make it come up again but it hasn't. It looks like the ad server cycles through the same set of ads for some time and then goes on to different ones.
When it grayed out the screen for it's popover, it persisted. Ahhh, it's called a
Lightbox in web design circles. It persists until the user clicks an X button in the upper right corner. Very obnoxious. But as I mentioned, I'll keep looking for it and get a screen shot for you.