Returning something to the rightful owner

Thirty years ago, when I was a child, I found a gold necklace. My sister took it from me and kept it. Two months ago she returned the item back to me saying she is not entitled to it. The local police law states that if you find something you must hand it in to a local police station and if no one claims it within 28 days then it becomes legally mine. I handed it in to the local police station and no one claimed it. I couldn't say I found it thirty years ago because I might have got into trouble for keeping it so long. Obviously no one claimed it so the police have said I can keep it. I don't want to be sinful so what should I do with the necklace?