Search Google for the "secret" URL. Make sure you click the option to show the "omitted" search results. In the search results, look for the results that date from before the article was posted. You will find an URL on pastebin.com. Look at the timestamp. This page contains a partial HTTP log, containing not only the relative URL but also, in the referrers, the complete "secret" URL. So, my conclusion is: in the group conducting the "secret" chat, somebody posted the HTTP log to Pastebin.com, and then, and only then, was the "secret" URL picked up by Recorded Future.