The canyon in Ghost's Domain intersected with multiple other canyons, and there were multiple twists and turns to them. Every 328 feet of the canyon will have a small entrance resembling a small valley.
If the entrance to the valley had a plate in front of it and had the sounds of wind signalling fierce fighting coming out from it, it meant that there was already someone training inside, and many people will not enter the place to prevent causing unnecessary conflict. That was because visibility inside was poor. They will have to prevent the people inside from attacking them, and they also had to be on guard against the traps that might be in there.
Usually, players came here to train, and they were not there to cause trouble. As long as the party was not incredibly powerful and domineering, no one will stir up strife by choosing places that were already occupied.
It was the same for Happy and the others.
