A more likely scenario is that this is a prototype of a four-door Converj. The Chevrolet Volt-based Converj appeared at last year's Detroit Auto Show as a two-door coupe with a similar profile to this car. The car in the photos appears to offer similar proportions to the Volt.
It's certainly possible that Cadillac has shifted design directions for the ATS to go with this fastback look. It's also possible that Cadillac is developing a five-door hatch based on the ATS for the European market to go up against the Audi A5 Sportback and the Gran Turismo version of the next generation BMW 3 series that was recently spotted testing. Thanks to Ed for the tip!
[Source: Burlapp Cars]