Not all hidden treasures are buried in the ground. I've seen many stories of treasures being found inside mattresses when an old person died, or inside walls when an old house was torn down.
In this show, I review a book that covers indoor hiding places. It was written to help you hide valuables inside homes or vehicles, but with reverse-engineering it becomes a book on finding cleverly-hidden treasures.
Several books have been written on this topic, and this one (The Big Book of Secret Hiding Places, by Jack Luger) is pretty representative of the bunch. If you're ever asked to search a house, this book will give you some ideas you would never have come up with on your own.