What do you do about a copy of the Qur’an that you have, and you strongly suspect it was stolen from your hometown mosque before you reached puberty, but you are unsure of that?

If you suspect a Quran copy was stolen, the default is innocence. However, if you are certain or highly assume it was endowed for the mosque, you must return it immediately and repent and seek forgiveness for delaying its return. If you took it before puberty, there is no sin. However, keeping it after puberty is a sin that necessitates repentance.