I've never run into this and hoping someone has an answer?
A client has lost ranking lately and there have been no warnings in Webmaster Tools.
What I have seen in the logs is a large increase in the number of visits from Russia and neighbors. The referring sites are completely unrelated to my clients business and range from seemingly legit businesses to porn sites.
I have viewed the source code of the referring page and see no link to my clients site but the traffic continues...
Could traffic like this effect rankings?