I have my 3 year old Granddaughter here 3 days a week. If she has an afternoon nap I sit with her and read a book. I had to answer the phone in the other room once, and she woke up alone, and was quite frightened. I wouldn't leave any child, especially one that young, alone at night. In this country it would be illegal. If that poor little mite woke up, and couldn't find her parents, she would have been very frightened. If someone passing had heard the child crying, knocked at the door and got no answer, they would realise immediately that the child was alone.
There have been cases of children in Europe being stolen for adoption in other countries. There have also been cases of children being abducted by paedophile rings in Belgium, and smuggled out of the country. It is never safe to leave children unattended anywhere.
The parents are both Doctors, so obviously well educated, and they should have known better.
I hope the poor little mite is found safe and well, but it seems tragically unlikely.