This latest prototype has a different front fascia with larger lower air intakes compared to earlier prototypes. It also wears larger alloy wheels, and seems to be riding on lower suspension and utilising bigger brakes.
Out the back, this vehicle features the same exhaust setup as earlier cars, rather than the typical quad-tip arrangement seen on true M models.
Unfortunately, as this car was running in a fleet of cars, including an M5, the spy photographers were able to discern what type of engine this prototype is powered by.
It's unknown if this prototype, and earlier ones too, are pre-production versions of the new 6 Series or 8 Series. Rumours have been circulating that BMW will replace the current 6 Series with a new 8 Series model, which will be pitched as a sporty luxury grand touring two-door.
BMW has even gone to effort of trademarking the 825, 830, 850, 860, M850, and M8 nameplates.
According to this scenario, in time, BMW will relaunch the 6 Series with a new generation that will be pitched as more of a sports car, and a competitor to the Porsche 911.
Either way, the car seen here, given its dimensions, seems to be based on the latest 5 Series, and will serve as the effective replacement for the current 6 Series coupe.