I can't think of anywhere to mount an air horn without it standing out. The Stebel Nautilus is probably the most compact, self-contained air horn on the market, but it is still a bulky item to try to conceal on a naked bike. I would love to have one because they are incredibly loud. I always enjoyed watching people do the safety inspection on my V-strom which had the Stebel well hidden behind the fairing. Always made the guy jump when he thumbed the button.
To me, the stock horns are decent enough, but there isn't such thing as too loud.