Official NASA Caption: S121-E-06211 (10 July 2006) --- Astronaut Michael E. Fossum, STS-121 mission specialist, removes the trailing umbilical system-reel assembly (TUS-RA) from the S0 truss on the International Space Station during the mission’s second session of extravehicular activity (EVA). Fossum is anchored on the mobile foot restraint on the Canadarm2.
What are we looking at? A mysterious object can be seen in the background of the photo. It does not appear to be a satellite or other "space debris".
High resolution photo is available for download below.