Stars Beyond Reach:Surface Scouting
Thanks to ElOhTeeBee for writing the below:
Your scouting ships leave microsensors seeded in their wake that, while harmless aside from the intel they gather, are impractical to clean up due to their tiny size. It'd be like finding a needle in a thousand haystacks, over and over again.
This is why you can't see stuff you haven't scouted, but you can always see stuff you have.