Google may have a new web crawler that specifically looks for Favicons, the little icons you see in your browser bar next to the URL. Favicons are associated with a particular website or web page and can be customized by the site owner.
Valentin Pletzer posted on Twitter that he spotted a new GoogleBot useragent that specifically is named for Favicons. It is named:
Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/49.0.2623.75 Safari/537.36 Google Favicon
It comes from the IP range of 66.249.93.x but it is no where listed on Google’s crawler info help document.
We know Google has shown favicons in the search results over the years. So maybe this is how Google indexes these favicons?
Forum discussion at Twitter.