It possibly could be a ground wire if it has a loop on the end of the wire. Sometimes a small screw will connect a wire to the alternator for grounding purposes. Take a test light with the engine running and see if this lead is live or dead. If it is live it is not a grounding wire so tape the end of it for now. If the car is charging properly and all systems are working and you are not experiencing any problems with the battery I wouldn't be overly concerned as sometimes there are extra wires used for testing components.

