Here's the responses from a couple of the admins:
I assume it is something to do with the Google adWords adverts that appear. If the member has his security settings too high then he will probably get these messages.
Apart from that, there are no security certificates for PPRuNe. They tend to be a functon, if I'm not mistaken, of secure websites, ie. those where the url begins https://
Yes, they are to do with SSL sites - commercial online sales sites. The Equifax cerificate is issued by GeoTrust. If the member is using a tabbed browser, such as Firefox, Opera or IE7, it may well be that the pop-up refers to page open on another tab. I think it's unlikely that it would be generated by any of the ads on Pprune unless they were clicked on by the member.
HTH
SD