Nightingale Legacy Portal
While going through the gloom site of power, i found 2 lore hope echoes and I was hearing a 3rd one but I couldn't find it. I ran around a bit to try and pinpoint it by the sound and the sound is the loudest when i'm next to a wall at the bottom (where the contraption that hold the card is). It sounds like the hope echoes is on the other side of the wall. To be more precise, it's behind the wall just behind the slope that leads to the bottom.
After some wall hugging, jumping, climbing and grapeling, i managed to find a way to go outside the walls (under the ground) and i can confirm that the hope echoe is indeed "inacessible" behind the walls.
To go there, I found a crack in the ceiling at the back of the room and i grappled trough it. That's how I managed to get outside the map.