I've never found one, but if I were to make a cache, it would have to have the following characteristics:
1. It would have to be in a place where I could access it without being observed.
2. It would have to be in a place where I would not be noticed coming and going, that is, a place I would frequent anyway.
3. It would have to be in a place that other people did not frequent, and could not come and go unnoticed.
4. It would have to be easily accessed for deposits and withdrawls.
5. It would not necessarily be buried.
Thus, under the anvil in the barn, in the rafters in the chicken coop, behind a loose stone in the fireplace (inside the house, and not in the firebox) and thousands of other places would be suitable locations. :P