On Gen I and Gen II that is the threaded hole where a rear view mirror could be attached. If it is capped I would expect it to be threaded. The right side used to be threaded and capped too, but starting with the '04 it was never even drilled. I have used this very handy mounting point for my heated grip controller and my cruise control pad. Neither of which would be necessary for your Gen III with them being built in.
Edit: Iggy and cross posted at nearly the same time.