There has to be a lot more to this story. Yes, what they did is monstrous. But I have to believe these folks were just a bit nuts themselves.
They had an 18 year old daughter one day, and then no one sees her again?
I know Japan has some unusual cultural things going on...but the whole thing sounds just too nuts.
The house looked like a criminal compound with high walls and surveillance cameras to scare off potential intruders. Definitely not a normal home. Why the police or neighbors or school officials didn’t inquire further I have no idea.
If she disappeared at 18 everyone probably assumed she was married off, left home on her own, or was institutionalized. If she had been known to be mentally ill- perhaps schizophrenia- before the parents did this, and neighbors just assumed she had been sent away to an institution, in that culture, maybe mental illness in the family is as bad a loss of face. It may have been considered polite not to inquire about how she was doing or where she had been sent and embarrass the parents.