This feature was discovered by Rota, whose website is called "The Secrets of Mars."
The image comes to us from the Spirit Rover. This is my color processed version of the L2L5L7 frames:
The feature is in the middle of this cropped cut of the color version:
Here is a youtube clip of the area, showing interesting features on the back of the sphynx:
And here are some true 3-d renderings of the scene:
With darker contrast, for comparison:
Special thanks and credit to Rota for his discovery and work on this feature. Visit his website at "The Secrets of Mars." Also, special thanks and credit for the original images taken by the Spirit Mars Exploration Rover to: NASA/JPL/Cornell.
Raw Images Courtesy of NASA and ESA. Special thanks to EVERYONE who has contributed in the search for Mars anomalies. Also thanks to Google, Picasa, ImageShack, and Photobucket.