They are more likely living in somewhere relatively dense in population, and it is easy for people to take notice something weird going on with noise, and more often, report of odors afterward. (Unless they go out somewhere in the middle of the mountain, which happens; not everyone having a car can make this challenging option.)
Another factor is their mindset, too. While most of Japanese are not religious, they do follow certain cultural customs from Buddism and Shintoism, etc.
Actually, I've heard a many cases of Japanese news that reveals the existence of the body in the ground because someone buried the body in their yard or in the ground under their house, etc. I'd still think it'd be challenging (and probably very unpleasing) to do the same while they are in possession, but at least they can have some effort of it contained.
Another factor is their mindset, too. While most of Japanese are not religious, they do follow certain cultural customs from Buddism and Shintoism, etc.