*UPDATE*
FarmHands and Arborists are confirmed to be inside the locked Mystery Box as well.

It appears that as of tonight, some very select special users of FarmVille can find secret locked Mystery Boxes while plowing their farms. These Mystery Boxes are locked with a code that is found via e-mail from FarmVille. This feature WILL make its way to all users eventually.

Rumored contents of this Mystery Box include golden chickens, Mammoth Statue and Totem Pole. More news as it comes!

Source: FarmVille, FarmViller