An as yet unknown number of keys for Sniper Elite 3 have been stolen, resold, and subsequently cancelled.
In a thread on the Steam forums, developer Rebellion explains the situation:
We have been saddened to learn that some fans have been caught out by companies selling unlicensed or stolen Sniper Elite 3 Steam keys. If you have been affected by this we encourage you to contact the seller in question and seek a refund immediately.
Rebellion says one of its retail distributors alerted them that some of the Steam keys were stolen, and those keys then started showing up on Steam. Rebellion asked Valve to revoke those keys. Rebellion says that it doesn't hold the retailers that sold the keys responsible, as they likely bought them believing they were legitimate.
Neither Rebellion nor Valve are offering refunds for the keys, however, and those affected have to contact the retailers in question.
Rebellion has offered free keys for the Target: Hitler DLC for those affected, but to use those keys, the users have to have a valid copy of the game. It's likely the best a mid-size developer can do in a situation like this.
While the details on who stole the keys and how are still unknown, this is an unfortunate, frustrating situation for all parties. The developer has to recoup the development costs and can't afford to have a bunch of unsold copies in users' hands, the legitimate resellers of the keys are stuck with the responsibility to offer refunds, and users may have no recourse if those resellers choose not to.