Heh, yes - as soon as Amara said that line, we knew how Mary figured in; but before that we didn't guess. --though thinking about it, Sam & Dean hugs usually happen after someone dies and comes back (All Hell Breaks Loose, Mystery Spot, Lazarus Rising, the 8th season premiere)(actually this might be the first time they hug before going off to die? There was no hug in Swan Song or the 7th season finale...) So maybe that was a clue!