[Den-O]

I always just assumed that the Black Star Dragonballs had never been found via radar before because they were at the lookout and perhaps the radar as a piece of technology wouldn't be able to detect such a mystical place. That or the Black Star Dragonballs didn't emit the same type of signal for detection as the normal balls. Pilaf had also created a device at least once that could interfere with the electromagnetic pulse the balls emit. Perhaps he did that?

Either way we don't see the radar used to detect them til after Bulma upgrades the Radar for use in the wider range of open space. So it's very possible that they do emit a different signal that was only detectable after that upgrade.