It seems hard to believe that someone would just leave 20,000 untouched for 22 years in a bank account. It seems really extreme that she would go to such lengths to hide from her family. It seems more logical to send them a letter that she has started a new life and does not want to be contacted. Instead the path she has chosen has launched a huge investigation that she wouldn't have wanted but set herself up for. Why not just tell the police I do not want to be found, please tell my family?
I do think she was hiding a relationship from her family though. As to why, maybe the request of the F. Remakel. The one who was probably in the dark car, the one who was coming over when Marion made Sally's husband leave. The one she met from the local ad. She knew French, she reached out to him because she was lonely. Maybe she fell in love and he convinced her to sell everything and go explore Europe with him. Then the last phone call she said she was going to be out of touch. Not because she was going back to Australia, but because he was taking her somewhere in Luxembourg where she would not have any way of communicating. Why call your daughter and tell her that if you didn't care about her anymore and wanted to start a new life? Why not just start a new life? Does not seem like she wanted to vanish at all to me.