Still here. I received the popup twice prior to logging in (connected from a link in a thread subscription email) and once after logging in. I checked the UCP and couldn't find an option to turn it off -- there should be an option there somewhere.
Regarding where the popup is coming from, it's from the web server. It has nothing to do with any software you do or do not have installed on the machine running the browser. You can see this pretty clearly if you watch the HTTP network traffic. (I'm running the DD-WRT firmware on my router with an extension that can dump network packets.)