Thanks

ORIGINAL: Shark7
One other thing to keep in mind is that Exploration ships run at the first sign of trouble. Meaning they may move into a system, find a pirate or space monster there and quickly leave the system. This opens the system so you can see everything there, but since the Exploration Ship left in a hurry, the stuff didn't actually get explored.
ORIGINAL: SirHoraceHarkness
Upgrading the scanner range will solve most of this as it will have a larger trigger radius so it can detect more stuff as it goes along its way. So if a planet has other items to detect they will usually fall into its detection range as it approaches. I think the default range is tiny so that might be the issue.