Snipped. This is something I have thought about as well. A father in prison, or perhaps even he was once there and that’s the only information the mother had. Maybe she remarried and had other kids. I can see an arguement, like “Im the only parent you have, dads in prison” and a teen thinking the grass might be greener. Perhaps dad was no longer at that prison so when they asked, there was no recognition.
The same could happen if dad was a guard or employee. Never made the connection that it was his child. Maybe never knew about her or last saw her as an infant. And typically we deny or divert our thoughts. It wasn’t uncommon in those days for mom to get custody, remarry, and the bio dad had no rights or never saw their child again.
Even if she had a prison pen pal, they might not have acknowledged the situation if they discovered she might be a minor. Never heard of her...especially if they were a predator to begin with. Or a married prison guard. But, the fact she could be underage most likely limited acknowledgement if there was romantic involvement.
There is also the possibility that she did have a destination in mind, but only used the prison as a geographical reference point to get to her destination.