Pathfinder: Wrath of the Righteous contains a ton of secrets and some of them are not immediately apparent, like the heavy breathing in a certain prison.
Ineluctable Prison is closely entwined with the story of Pathfinder: Wrath of the Righteous so there may be some spoilers ahead. Then again, if you specifically searched for this topic, you probably completed that content already.
Anyway, while you're wading through Baphomet's labyrinth, you might go outside its walls to the northwest at some point. This is where you will find an interactable object that merely says you can hear heavy breathing but can't see anyone. Using True Seeing or See Invisibility will not help.
Instead, you will need to fight the final boss of the labyrinth and this individual is located in the eastern side of the map, as shown on the image below. When you're done with the boss, you will simply need to trace your steps back to the wall and you will find a certain character pacing around the area.
In order to avoid spoilers, we will not say who the character is, what state they are in or what to do with them. This is a roleplaying game after all so all these choices are down to your wishes but potential consequences may not be apparent just yet.