Honestly I am not sure whether this is a bug or expected behavior however I was asked to make a bug report for it.
When PrivDog removes an ad on a website, it doesn’t always replace the ad. It seems to be website specific (or ad-network specific) so if PrivDog once replaced the ads on that website (or for that ad-network) it will continue to do so and if it didn’t replace and only removed the ads then it will continue to do so.
Example of website where third-party ads are removed but not replaced with ads from AdTrustMedia: http://www.sweclockers.com/ The third-party ads that I’ve seen are from adtech.de which PrivDog then removes, however PrivDog does not replace the ads with ads from AdTrustMedia.
Example of website where third-part ads are removed and replaced with ads from AdTrustMedia: Pixie Trix Comix - Eerie Cuties - The third-party ads that I saw on that page comes from googlesyndication.com and if PrivDog is enabled it will remove those ads and replace them with AdTrustMedia ads.
No, this is not a issue. As you can see from PrivDog frame, we have several categoryes for blocked threats, such as Trackers, Statistics and etc. Also we have some “inner” list of networks, which we never want to be displayed and replaced, just block. Because sometimes, that ads may be presented on very bad sites, like adult or strong malware. And you already have found one of those.
My observation is that Privdog does not replace blocked ads a t all but simply blocks those that are not trusted and allows the trusted ones through. On my Yahoo home page, a few ads get blocked and are not replaced by anything. The big banner ad for Netflix is allowed through.
I only know for Chrome since it’s the one I’m using, right-click the ad and click “Inspect element” then it will take you to the code which should show the url for the ad, if it’s from AdTrustMedia the url will be theirs.
Another way is to check for the “AT-M ad” watermark thingy on the ads, however that could probably be faked by malicious ads so I wouldn’t count on it, the “Inspect element” method is better for knowing where the ad is from.