I haven't seen this issue before, so I'm hoping someone here can give some insight.
I have a client that I did a website for, and has been up 6+ months. Recently we found that when doing a search for the site, more specifically with Yahoo/Bing, that instead of their site title, it shows something for some sort of drug. When you click on it, it goes to my clients website and nothing out of the ordinary. If you click on the cached, then it comes up with extra info in the footer and the title at top of browser is for this drug.
I have gone through all the source coding and everything is fine. When I look at the source code from this supposed cached page, it shows the extra coding.
I have contacted Yahoo about this, and they say that they are investigating it and have determined that it is an issue on their end. At this point, I'm waiting for Yahoo to find and fix their problem.
The website is ICTOA (International Counter-Terrorism Officer's Association). If you search ICTOA, you will see what I'm talking about.
Check your source code. Sometimes hackers can manage to add encoded advertisements and specify that they only show up in the site's source code when the site is hit by a web crawler, but not with a browser.
I've seen this using the base_64 function in PHP.