I see in my special tracking software that the Google engineers are almost daily on two of my sites for hours. My software shows exactly where my visitors come from and what search queries they used and in which major search engine.The Google engineers were over 100 times on my sites already over a 7 month period. I think it is because I got for my one site over 3000 keywords in the top 10 in Google SERPs. The other one over a 1000 keywords in the top 10 in Google SERPs.Some of my keywords are number one for almost 7 months now from tens of millions of other websites.Have you experienced a lot of visits from Google staff on your sites?