Hackers sometimes add strange, off-topic hidden content and links to questionable pages. If it’s hidden, you might not even notice it right away; but nonetheless, it can be a big problem. While the Message Center may be able to give you a warning about some kinds of hidden text, it’s best if you also keep an eye out yourself. Google Webmaster Tools can show you keywords from your pages in the “What Googlebot sees” section, so you can often spot a hack there. If you see totally irrelevant keywords, it would be a good idea to investigate what’s going on. You might also try setting up Google Alerts or doing queries such as [site:example.com spammy words], where “spammy words” might be words like porn, viagra, tramadol, sex or other words that your site wouldn’t normally show. If you find that your site actually was hacked, I’d recommend going through our blog post about things to do after being hacked.
There are a lot of issues that can be recognized with Webmaster Tools; these are just some of the more common ones that we’ve seen lately. Because it can be really difficult to recognize some of these problems, it’s a great idea to check your Webmaster Tools account to make sure that you catch any issues before they become real problems. If you spot something that you absolutely can’t pin down, why not post in the discussion group and ask the experts there for help?
Have you checked your site lately?

