> It's visible in the visible spectrum and hearable in the human hearing range

Sure. It's just that radar lets you see tens or hundreds of miles away, and visual doesn't. And sound lets you hear where something is only at the speed of sound, which is less useful for something that can fly faster than the speed of sound.

So developing a weapons-quality track from visual and sound data is problematic. That means that, while not invisible, it's "invisible enough".