** Spoilers and speculations ahead **
First clue- The episode title – Book of the Stranger and The Hound named his horse Stranger. Also, in the books, it is strongly suggested that he’s alive and rescued by a group of people sworn to the Faith of the Seven. Since Stranger is also an aspect of the Seven, it gives us another reason to believe that the title hints at The Hound’s return.
Second Clue – Rory McCann was spotted in Belfast last year and Ian Mcshane has almost confirmed that he’s bringing back Sandor Clegane so he should appear at some point this season. Check out his interview here.
Third Clue- This Instagram post by Hafþór Júlíus Björnsson (The Mountain) asking if we’d like to see Cleganebowl. Duh!