A new key has been spotted on Randy Pitchford's Twitter page. This key contains 1 golden key. The expiration date is unknown so redeem them asap.
The key can be found at the following discussion post: http://gtm.you1.cn/groups/borderlands-2/discussions/1/846960628361184810/

If you haven't already, subscribe to this forum to get steam notifications every time a new shift key is released: http://gtm.you1.cn/groups/borderlands-2/discussions/1/