Yeah. It's too crazy. I've gone over the possibilities in my head a million times. If it was a person - say a peeping tom - who saw the parents leave, how did they get Heidi out? She was 5'4" and weighed 145 lbs - the size of an adult woman, and I'm sure could have put up a fight. If it was a burglar, why didn't they flee for fear of being seen and caught? I feel like most burglars don't also abduct children on a whim - those are two different types of criminals. If it was a person who lived in the building, why did they accidentally break open a window that led to a shared bathroom on the first floor but didn't connect into the Morningstar's apartment? If it was a person at the scene of the accident or otherwise saw the Morningstar parents there, were they just so overcome with a desire to kidnap that child that they raced to the Morningstar residence?
Let's build out a circle of who knew that Heidi was alone:
Center - no inferred information: Mr. and Mrs. Morningstar knew she was alone.
Center: An unknown person, prowling around that particular building at that specific time could have seen the Morningstar parents leave the property while leaving the children alone and therefore known that the children were vulnerable.
Ring 1: A person in the building who was awake and heard the Morningstars leave likely could have inferred that the Morningstar children were alone.
Ring 1: A person who lived on that block who knew the Morningstar family and was awake at that time and saw the Morningstars leave could have inferred that the children were alone.
Ring 2: Some of the people at the scene of the accident likely could infer by the presence of Mr. and Mrs. Morningstar that their children were likely alone.
Ring 3: A person that was called by a person at the scene of the accident and was told that the Morningstar parents were present.
I just don't know.