I apologize for missing this a week ago, when I was sitting in the Google hangout where Google's John Mueller went on record that Google uses unlinked URLs or domains in body content as a way of discovering new content and pages.
John Mueller, a Google Webmaster Trends Analysts. said in a video 47 minutes and 32 seconds in (I was distracted with something else), that Google uses these URLS, even though they are unlinked, to discover new pages. It does not influence rankings but it does influence discovery.
Here is the video, skip to 47 minutes and 30 seconds in:
Credit goes to ignitevisibility.com for spotting this first.
Also, Dan Petrovic tested this a year ago and came to the same conclusion.
Forum discussion at Google+.
Image credit to ShutterStock for robot turtle.