In the books, there is a Gravedigger on the Quiet Isle who Brienne sees. He's bigger than Brienne, covers his face with a scarf, new to the monastery, has injuries that are healing, and Sandor's horse is seen on the Isle.
The working theory for a lot of readers is that at Cersei's trial by combat, Cersei will name Robert Strong/FrankenMountain and the Faith will name Sandor, leading to Cleganebowl.