Could this be the 2013 Mustang that will be unveiled at the L.A. Auto show in November. Its possible as TheMustangSource forum member Stangpilot007 was lucky enough to spot a test mule roaming the streets of Dearborn on his way home. From the looks of the photos you can see the rear diffusor is painted unlike the 2011-12 Mustang GT, also the hood is raised and the rear fascia seems to be pitched more toward the front of the car. Another observation is the exhaust, it looks very similar to the Boss 302 with the large tips.
You never know, this could be the 2013 Shelby GT500. Check out the photos below and let me know what you think in the comments.