Maybe it's a picture hanger? No, maybe a garbage bag.

I love that NASA is now obsessive about the space junk. Not only are they combing over the heat shields with cameras multiple times looking for damage, but they are also peering out the windows looking for junk that might scratch the tiles when they start to leave orbit. Apparently the junk is rather hard to make out:

A second mystery object was spotted midday Tuesday and photographed by astronaut Dan Burbank. Jett said the object looked like a picture hanging clip. But it may be a garbage bag, which would unlikely be a damage risk, but the issue will be moot if the heat shield looks good, Hale said.

Now who was hanging a picture in space and forgot to pick up after themselves?