Starting tonight, a very small percentage of farmers will have the chance to get a special locked Mystery Box. Here’s information on how it works:

The Mystery Box can be found while plowing plots of land on your farm. If you find this special Mystery Box, you’ll get this popup:


Upon removing the Mystery Box from your Gift Box and placing it on your farm, you can select either Get Passcode or Enter Passcode from its menu. If you have not signed up to receive emails from FarmVille, selecting Get Passcode, will give you the following popup:

Note: If you have already signed up for email updates, the popup will ask if you would like the passcode emailed to you.

Once the passcode is emailed to you, you can select Enter Passcode to unlock the Mystery Box. You gift will be delivered to your Gift Box upon opening.

To reiterate, this feature is only visible to a very small number of players right now. Eventually, all players will be able to have a chance to receive this Mystery Box.

(Source)