We talked about this in the General Overview, and frankly, there isn't much more to add to this other than what I already said. Therefore, the below text is copied from the other chapter:
The Ghost Room, which you can probably guess why it's called that, is the room that the ghost hangs out in primarily. This room, once you find it, is going to be where you get most (if not all) of your evidence.
The Room can be anything from the basement, upstairs master bedroom, bathroom, kitchen, living room, or... you get the idea. The ghost even used to be able to chose the Hallway as it's room which means it has a huge area and it also means your job is going to be really difficult (however, I think they removed the ghost's ability to do this in the current game) but it used to be a massive pain. Can you imagine trying to gather evidence when the ghost chooses the hallway in Asylum?!
The ghost room can change mid-game, so if you're not experiencing anymore activity, it might have moved.
It's also important to know that the ghost room is where most but not all ghosts will spawn when they begin a hunt. Therefore, it's a good idea to not hang out here too long.