Official NASA Caption: S122-E-011016 (18 Feb. 2008) --- Backdropped by the blackness of space, the International Space Station is seen from Space Shuttle Atlantis as the two spacecraft begin their relative separation. Earlier the STS-122 and Expedition 16 crews concluded almost nine days of cooperative work onboard the shuttle and station. Undocking of the two spacecraft occurred at 3:24 a.m. (CST) on Feb. 18, 2008.
What are we looking at: In the background of the photo, an object appears to be moving at a high rate of speed. How do we know? Blur effects caused by CAMERA motion occur when the camera moves while the shutter is open. The result, is everything has a 'trailing' effect.
However with this example, you get a perfectly clear space station photo, yet an object appears behind it with motion blur -- giving the clue that it might be some sort of structured object moving at a high rate of speed - not a result of camera movement.
High resolution photograph available below for download.