rainborick - 3:56 am on Aug 17, 2013 (gmt 0)
tangor's got it right. In fact, the idea that search engines couldn't or wouldn't crawl <frame>d sites dates back to the last century. Whether or not it was ever true, search engines have been able to crawl and index frames of all kinds for at least the past decade. Frames do bring challenges for search engines, and Google has changed it's methods from time to time over the years. About two or three years ago, Google announced that they would sometimes consider the framed content as being a part of the parent <frameset> page and they will often use the parent's URL in the SERPs when it's really an internal page that ranks in order to prevent users from landing on a page with no navigation. I think the OP's scenario illustrates that practice. It's not surprising that all these machinations make it difficult to diagnose the situation from the outside.