I have a 6 year old and 3 year old grandson, who live with me. My 6 year old grandson has Aspbergers and is 'somewhere on the spectrum' per his psychologist. He is a wonderful child. He also has issues at times.
If it is a normal day, he is perfectly able to be left to 'his own devices' for a 1/2 hour or so, as he knows where to find me or Mom. He will sometimes sit at the table alone to eat, if he is taking his time, or watching a video or cartoon while eating.
He knows right from wrong, safe and dangerous. He is all boy, but is also responsible enough to know how to behave.
If it is a bad day, no holds barred, he can't be alone that long, period.
It sounds like Rebecca was around Max regularly. I think she knew his capabilities ans how well he needed to be 'watched'. Rebecca could have been doing a number of things. Where are the washers and dryers. Maybe she went to the bathroom and was then cleaning it. Maybe she didn't feel well and actually needed to be IN the bathroom for 10 minutes.
In the beginning of the autopsy it states she didn't see Max for 10 minutes. Later it says 10 to 20 minutes. If Max knew where she was, the situation should have been fine. Especially with her leaving him downstairs.