You really mean anything at least comet/asteroid sized, passing within the inner Solar System with a speed and an albedo that would make it stand out against th

You really mean anything at least comet/asteroid sized, passing within the inner Solar System with a speed and an albedo that would make it stand out against the background of stars (reaching at least say, apparent magnitude 8-10). Also, note that it should be found near the ecliptic plane, to improve the chances of detection. Now, exchange that for a black-ish ship a few hundred meters sized, quietly coming from Vega to visit say, Saturn. Nobody would probably notice it coming and going, despite Saturn being one of the most popular targets of the entire sky.