Just because carmakers don't publicize specifications of future models, doesn't mean that they're not obliged by law to register prototype vehicles created for development purposes if they intend to use them on public roads.
In some cases, depending on the country and local rules, they have to provide specific details. If we are to believe a report from the BMW loving crew over at the F30Posts, one of their members discovered a registration document for an M4 Coupe test car through a VIN check (there no mention on how and where).
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