When I was a teenager, I used to work for a construction company doing renovation of 100+ year old houses. I was mostly doing demo work, taking down lath and plaster walls with a sledgehammer, etc.
One day, we found a sealed metal box inside one of the walls. The foreman came and took it away. To this day, I wonder what was in it (and if the foreman ever gave it to the homeowner, for that matter).