Oh, a few things.
1 - My great-great grandmother died in childbirth. She had been raped, and whoever did it fled to another state, then she died. As soon as the baby was born, he was given to his mother's sister to raise. She already had quite a few children of her own, so she raised him as her own. It wasn't until her deathbed that he found out his birth mother had died. That threw a rather large wrench in that side of the family tree.
2 - I had a full sister. When I was about a year old, my parents had another child, a girl. She apparently died very young. The body was cremated, and no one knows what happened to the ashes. My mother went a little crazy, destroying any pictures of the baby, including any pictures of her when she was pregnant. Two years after the missing baby, they had one more kid, my full brother. I only found out about my sister recently, and to this day, my mother denies it ever happened.
3 - I have an uncle out there no one in my family has met. When my maternal grandmother was 12 or 13, she was raped. She got pregnant (which was a scandal back then - it was HER fault she got pregnant, not the rapists!). When she was ready to give birth, her father took her into the hills, she gave birth in a cave. Her dad let her hold the baby boy, then took him and gave him to a family in another town. She had four daughters later, with my grandfather, but never had another son.
Since documentation was lax, at best, there is nothing for any PIs to go on. Two of my aunts hired one a few years after my grandmother passed away, but being in a farming community, in the late 40s/early 50s, no one would talk, or had any information. And of course, no hospital information.
So, somewhere out there, I have an uncle, and possibly other relatives, that no one knows.