Sometimes I see green dots, like something marked for hauling, which are apparently located deep below the surface.
Just now I've observed a sap planting a pine tree and for some reason the tool went flying, clipped through the ground, and is now below the surface.
The tool is marked for hauling and it results in the same green dot I've seen on earlier occasions.
So I guess all these green dots are items that clipped through the ground and are now inaccessible.