He was in the apartment and his mom went to the bathroom. She called the police so quickly that while they were responding in the neighborhood the bowling alley employee had already called in the kid in the machine.
She was no more negligent than any other mother with small children. Kids that age are smart, fast, and can disappear in an instant.
When I was 4 in 1955 my mother was paying for something at a store counter and I was at her side. Before she was done (less than a minute) I had seen a woman outside the open door that looked like my grandmother and I went running to see her. By the time I caught up to her down the street and discovered she was not grandma, my frantic mother was out searching for me but by that time a passing beat cop had picked me up, determined I was lost and took me up to the police station in a nearby building and gave me an ice cream. I guess I cried pretty loudly because I saw the jail cells with bars in the open area I was in and was afraid they would put me in one. My mom found me quickly and I learned the lesson that when you walk outside your sphere of security bad things can happen.
This is why you toddler “proof” the doors.