How did Mrs. Freeman come to Detroit as a little girl?

In Christopher Paul Curtis's Elijah of Buxton, Elijah is an eleven-year-old black boy growing up in a settlement for escaped American slaves in Buxton, Canada. He has never known the horrors of slavery, having been born free in the settlement after his parents, the Freemans, had already been living there.
When Elijah's mother Sarah was a young girl living in slavery her master took her with him to Detroit, Michigan, which is just across the border from Buxton. When she came back to her master's home, her mother was upset that Sarah didn't try to escape to freedom in Canada. The next time young Sarah was taken to Detroit, she knew that freedom was close and that she would not let the opportunity to run slip away from her again.