ranakay,
Well Sir, in my opinion, you need to get that $93 dollar an hour to do his job an that is, investigate just EXACTLY where he thinks that "extra" cable goes. We can all send suggestions about where we think it might be going but, that does not do you any good. There are so many possibilities of where it might go and for what purpose, it would be really difficult for us to accurately tell you what it might be for.
Many electrical problems are hard to diagnose but, simply tracing a cable, from a battery post, is certainly not that difficult.
Scott