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Just because there are no spyshots does not mean a concept does not exist.
BMW have multiple testing centres where cars can test in privacy even without disguise away from a lens.

Planned concept cars in preview-production phase do not even show even on intranet spreadsheet until after a concept has been revealed.
Look at last years X4 Concept as an example. Palbay got the first spyshots around July-August.

Existing variants of current cars can hide under the bodies of that same member of the family and go unnoticed but still be able to be evaluated as that model.
Before final prototypes initiate public testing.

The proposal for Gran Coupe is based on the F20 so there is flexibility to use that extended platform. BMW are eyeing global reaction to the competition before striking. The car has already been designed.