You can hear things that are going faster than the speed of sound, so I don't see why something that is outrunning its own light would be invisible. You wouldn'

You can hear things that are going faster than the speed of sound, so I don't see why something that is outrunning its own light would be invisible. You wouldn't see it coming, but you would see it leaving. If it's not transparent then you'd see its shadow, if it smashed its way through a planet you'd see the hole, etc.