Home alone : do you ever lock your child?

In the recent weeks, I've come to know a lady who lives in an adjoining building. She has a five year old son who I rarely see outside together. The kid never goes to the park or comes downstairs to play with other kids. It's all about school and classes in the evenings. There's no dearth of classes for kids these days from the time they are in the womb. No comments on why she doesn't want the kid to play or take him out to the park every evening. It's her choice, and I'm no one to offer an opinion. It bothered me just a teeny bit the few occasions I've dropped by at their place and the kid is all curdled on the sofa cranky - the reason for which was cited as "being bored". The few times he has come over to our place, I saw a stark contrast in his behavior - all chirpy, happily playing with LG - sometimes even very aggressively paying absolutely no heed to what his mother had to say.

What happened last evening disturbed me a bit. I met this lady at the tailor's and asked her where the son was - hadn't seen him for over a month. She mentioned he was sleeping at home. I assumed she must have had relatives visiting or that someone must be home. For some reason, she mentioned a few minutes later that she got about two hours everyday while the kid slept when she went about doing her stuff. She was confident he never woke up before two or three hours and slept deeply. At first, I didn't believe what I heard. But it slowly sank in as an eccentricity. About half an hour later, I reached home. While LG was playing in the car park, I suddenly heard loud screams of a child crying. When it persisted, for some strange reason it occurred that it could be the child in the next building. Just to make sure it was him, I shouted out his name. He barely got his head above the balcony - another mistake. I've no idea what the lady was thinking to leave the balcony door for a kid his age. He cried again...I felt helpless not knowing what to do. We looked around the neighborhood to see if his mom was coming but there was no sight of her.

I can never imagine doing such a thing with LG until he is grown up to handle himself even if I am doubly sure he'll be fast asleep for a while. As a child, neither me nor my brother had been left unsupervised at home until I was eight years old. Is this just me being paranoid or is this a common practice?