Squirrels are known to spy on each other and to steal each other's "investments." (Also, if one notices that another is watching, it will pretend like it's hiding the food here and then take it someplace else.)
Not just other squirrels. I have seen a magpie sitting on a wall waiting for a squirrel to finish burying its nut. Once the squirrel had finished, it descended to have a good poke around the burial area.
apparently this is why they will pretend to bury/hide stuff in order to fake out other squirrels.