"Not visible from an aerial shot with leaves on" doesn't really mean anything. If the roads aren't buried underground or in a tropical rainforest, they are probably visible to a satellite at the right time or wavelength.
New roads can be submitted by users. Google has a map editor where anyone can make changes and have them officially approved. They could have been submitted by a random Google Map user that was scrolling around and noticed missing roads.
Did the private roads require plan submission and building permits? The plans likely got stored in some government map database, which in turn got sucked in by Google.
