I don't think they dig with pick and shovel anymore, the shovel is usually to lean on while they watch the backhoe

Anything you put anywhere in civilization you run the risk of it being found during maintenance. Anything you rent you run the risk of it not being accessible in times of need due to possibilities such as power out, the building/business closed, etc.

Some possibilities I have seen are networking with others then I store a box for you and you a box for me. Another is getting permission to geocache on public land and place 6 caches to be found and one to not.