It was a secret no one knew, not even the person involved. The only sole witnesses are either dead or have chosen to forget the event entirely. So how does Harry find out what this secret is? Draco/Harry! This story is also called Family Secrets and The Cousin that No One Knew.

Or, Don't Look A Gift House in the Mouth. In the days, weeks and months following the final battle, can two men set aside their differences in order to guarantee the safety and survival of an innocent child? NC-17 HPDM

Harry finally gets tired of being mistreated by the Dursley's and his complaints ignored or dismissed by the faculty of Hogwarts so he runs away. He learns a lot on the streets and in the world he falls into.