So I was enjoying the atmospheric qualities of the Hunter's Dream, when I happened across this dialogue, which is probably Gehrman. As you can see, I only found it from noticing the Talk interaction, as there is no other sign he is there: no audible nor physical clue was to be found. I know the game has been out for a while, so I'm going to see if anyone else has found this. Feel free to let me know if it has!