That's one of the audio cables from the sound stage. If you look closely you can see the shadow of a boom mic on one of the rocks.
My first thought was: "Why would they need a mic? There's no sound in space" and then I felt the whoosh over my head. Well done.
This could be the cable from one of the harpoons that may have fired but didn't anchor themselves, or it could be a feature attached to Philae that's in the field of vision.