It is probably an earth strap to stop static build up around the filler neck I have one attached to my filler and it is held on by one of the bolts which you mentioned it probably fell off when the nut was removed or vibrated loose, if I were you I would stick it back on as the fuel area is not a good place to have a static spark occurring use a Nylok nut which won't vibrate loose, or use some of that Loctite Nut Lock stuff. RG