When Jaguar takes a car out to test run, they usually mask it under plastic, fabric, or other kinds of camouflage. By when a super-secret vehicle like their concept XFR-S supercharged sedan needs to be kept under wraps, they mask the entire test vehicle within another car. This is typically referred to as a "mule" car.

The drivetrain and engine of a test vehicle is placed in an existing model. For XFR-S, Jaguar used the body of a Jaguar XF at the track, that car is currently available at Jaguar/Land Rover Honolulu in Honolulu, Hawaii.

XFR-S is a supped-up RS version of the XF, and it performed beautifully testing on the Nürburgring when spy photographers caught it under the guise of a regular XF. Official details have not been released about the car, as it is still testing before hitting more world wide car shows and finally, dealer lots.

World Car Fans expects the Jaguar XFR-S "to be introduced sometime around late 2012 or early 2013."¹

Source:
¹http://www.worldcarfans.com/111081935858/2013-jaguar-xfr-s-mule-spied-for-the-first-time