According to an article Google may identify search engine spammers using historical data, it seems that Google is using historical data to identify search engine spammers. This is a warning to all the past spammers who tried the 'black hat technique' to falsify a site's search engine ranking.
You probably already know that engines are striving to deliver results based on your prior searches. But what a lot of people are missing is that search engines are building a database of every website’s, every company’s and every SEO’s optimization efforts.
As we speak, Google is building a very large rat trap and temporarily nourishing all of the rats who are eating the bait-cheese. Google wants you to join the various linking, advertising, and article-content networks so they can identify all the rat-trails and trap the rodents en masse.
Google is identifying former spammers in a neat manner. It saves copies of its search index. If at any time Google becomes suspicious of SEO techniques, it goes back to its archives and check what your site or what you did years back. Black Hatters beware, Google is on a prowl.