AND it was the first time that her husband was away all night for work. What a strange night for her NOT to lock the door, the first night her husband is away from the house. Hinky, imo.
Maybe the fact that he wasn't home to help with dinner, homework, baths, etc. led to her forgetting to lock the door. She may have just forgotten to check it because she was very busy and probably not used to having to run the household routine alone at night.
I have forgotten to lock my front door more than once due to various reasons when I was a single parent to 3 young kids. I'm not proud of it, and I do consider it carelessness on my part. But it does not make me a criminal. I was tired and overworked, and maybe she was the same.