A moratorium is in effect on adding information from Fire and Blood until December 20th.
- The Elder Brother of the monastery, while speaking to Brienne of Tarth, seems to know much about the mind and soul of Sandor Clegane. Perhaps too much for having only briefly spoken to him as he died.
- The gravedigger, along with all the other novices, wears a scarf which covers his face. This would have made it impossible for Brienne to have seen the distinctive burns attributed to Sandor.
- The gravedigger has a healing injury similar to the one that Sandor received at the inn at the crossroads.
- Brienne notices the gravedigger bend down to pet a dog, which many readers believe is a hint left by George R. R. Martin.
- The Elder Brother is vague while speaking of the death of the Hound. It is possible that he is speaking metaphorically and that he believes the violent and angry version of Sandor has died and now the new Sandor lives on. This is supported by the fact that the Elder Brother mentions that he 'died' on the Trident when he used to be a warrior. In reality his 'death' only consisted of a major life change (as he believes Sandor has taken).
- The presence of Sandor's black horse Stranger (now named Driftwood) on the Quiet Isle is another clue. It has been previously stated that only Sandor could lead or ride Stranger as the horse is exceptionally hard to handle.
- Some readers consider the fact that Davos Seaworth saw Sandor board a ship named the Prayer during the Battle of the Blackwater significant. 
It has been speculated that Sandor has been tasked with digging a grave for each of the people he has killed.
Evidence Against the Theory
The Elder Brother tells Brienne,
I buried him myself. I can tell you where his grave lies, if you wish. I covered him with stones to keep the carrion eaters from digging up his flesh, and set his helm atop the cairn to mark his final resting place.
According to this quote, it appears that the Elder Brother literally created a cairn and buried the Hound's flesh. Defenders of the theory believe that the Elder Brother could be being metaphorical, or perhaps have literally had to cut out some of The Hound's flesh while healing him; due to it having rot.