Most walls in our houses are made from a frame and a layer of drywall. That keeps the space between drywall sheets for electrical wires and plumbing pipes. That space could also be a place to keep money or important documents. Many times people hid something and forgot about it or passed away and never told anyone where to look for it.. Sometimes you can find someone’s forgotten treasure. People found real safes or old kids stashes from the previous century with baseball cards, diaries, etc.

Some things you find are not as exciting, for instance one of the most common thing found in old walls is used razor blades someone disposed of.

 There are several DIY projects on how to use the wall space to build alcohol shelves or hidden gun storage. There are always stories in the media where people found whole secret rooms in their homes. Some had creepy interiors and others just looked like kids' secret playrooms. 

One construction crew found hundreds of human teeth hidden inside a wall. But most times you find mold and old non functional insulation. And that's the creepiest secret.