"After Dempster, Marmol and now Sanchez, you may be seeing more ability than I do."
We know that the Marmol leak came from Marmol himslef, so that eliminates one of the names mentioned,
We know that the Dempster leak came from the Atlanta front office, so that eliminates the second name on the list.
There wasn't even a whisper of rumor that the Cubs were negotiation with Sanchez, so that indicates that the Cubs front office can keep things quiet if they want to. Just as there were few prior reports when they traded for Rizzo, Stewart, Dempster to the Rangers, Soto, and a handful of others.
It is quite possible that someone that had NO reason to leak, actually DID leak. But when one person has NO reason to leak information, while a SECOND person would make a lot of money leaking that same information, you are a fool if you bet on the first guy.