We aren’t all perfect, and we may have all done things differently, that's parenting.
At the end of the day, you know nothing about this, except what you've read and heard on the news which places you in no position to comment about the sensibility of the parents involved.
As for 'it should never happened', unfortunately it does everyday, we just don't hear about it as much unless the news front is quiet.
I deeply hope she turn's up fine, my only daughter is the same age and I keep an eagle eye on her but on a relaxed holiday, things are different, whether you like to admit that or not.
My only gripe is if the reporting turns out to be unexaggerated, half an hour is far too long for such a child to be left on there own, especially out of view.