Google sent out new message notifications to 20,000 web sites that are hacked. Head of the webspam team, Matt Cutts, announced on Twitter that they have sent out message notifications. Hackers may change the content of a page, add new links, or add new pages to the site. The intent can include Phishing to tricking users into sharing personal and credit card information or spamming. A year ago, Google began labeling hacked sites and sites with malware as sites that may compromised in the search results snippets.
Source: https://thehackernews.com/2012/04/google-sent-hacked-notification.html