According to Google’s head of web-spam team, some 20k sites are compromised in a way that their rightful owners may remain unaware of such a hack! The hack causes the redirects to occur only when visitors uses Google’s search results to visit a website. Google has sent out notification messages to all websites owners that were found to have compromised redirects!
Read more – http://searchengineland.com/google-sent-20000-hacked-notification-messages-to-webmasters-today-118585