Suppose someone went missing and no body is found, nor any other sign of what became of her. I believe in France it takes 10 years before a missing person is assumed dead when there's no actual proof of death (Wikipedia, but other sources appear to confirm it)
The missing person had two grandchildren as her only living heirs (husband dead, no siblings, children also dead). She adopted both of them when their parents died, and they lived with her at the time of her disappearance. At least one of them was 18 or older at the time.
Assume there is no sign of foul play by the two heirs.
They can't inherit until their grandmother is legally considered dead, I'm sure, so what happens in the ten years before she's declared dead in absentia? Can they keep living in the house during that time?