I'm not sure its hidden on purpose. There doesn't appear to be any styling to hide it, and when I went to the URL that the inner iframe was linking to (the iframe inside of the iframe), it gave a blank page.
I think Google banned the domain from Adsense. I had a neglected UGC site that gradually became inundated with shady links and content, and this happened to me. Perhaps a site with low clicks and repeated, short-lived ad serves is worth banning.