I think this was mentioned once before and someone said something about an email application. A quick web search in google of course revealed that gnome is a whole productivity suite that seems to have its origins in open source.
Taking into account the geeky origins of Google, I would not be surprised if the makers of the Google Adsense monitoring portion of the program created a system based on this architecture, which leaves a telltale sign as a referrer in our logs.
I would not be too worried... if you are a high earner? high volume? particular topic? or just happen to throw up some kind of flag.... you might get the visit. You can only do something about things that you have control over. Just make sure that those are covered and chances are you will be fine.