While going through my Google analytics data for one of my domains, I noticed a very strange URL as a referral. It was domain.com/this_page_should_not_exist.fake (no page extension like html, etc). Being a curious type and working in SEO meant a further investigation was on the cards. I checked the location of the visitor which was San Francisco, where Google has one of it’s offices. Next was the operating system used which was Linux, which is in the most part what Google employees are using, which is said to be a customized version of Linux Ubuntu.
Also the other strange thing about this saga is that the domain in question is not in English, so it would be very unlikely that a user from San Francisco, using Linux (used more o less by only 1-2% of the total users), visiting a non existent URL. All this leads me to believe a Quality rater, working for Google is testing various sites to see the server response codes when a user calls a non existent page (which should be a 404 of course), probably to see if any funny busy (Black-hat SEO) is going on.