For your caches you could improvise a metal detector. Attach those really strong rare earth magnets to the lids of the buckets. Use a smaller magnet/piece of ferrous metal on a string to locate the buckets. This, of course, assumes you have buried them.

The down side is if someone with a pacemaker happens to lie down on the ground over your bucket...he's a goner!!!

Feel free to chime in everyone.