[user104658]

Again - it's a genuine possibility that they sold the baby in an illegal adoption. That could either be to someone planning abuse (unthinkable) or to people who have been unable to adopt legitimately and were willing to do it by other means. It happens a lot more than is ever recorded. People sell babies without anyone ever knowing they were pregnant.

The latter is really the best case scenario here so we can hope for that. I think it's possible that no trace will ever be found and unless one of them talks, we'll never actually know.