I've done various scans on the site's file system and found one spambot, which is now quarantined.
The search continues.
"Things should be made as simple as possible, but not one bit simpler." ~ Albert Einstein
If the search is continuing, maybe people could help you. I am wondering, if any of these are targeted ads (inserted ads that people see, based on their cookies and browsing history)?
If someone sees one, could they clear their cookies and retest?