The invisible hills are nearly impossible to detect from the air, and even difficult to spot from the ground because.. well because they are invisible! HAHA
But seriously.. they aren't just invisible.. they also are 'X-ray' in a sense that all trees and buildings behind them disappear, but the ground and the roads still appear through the invisible 'window' that is the hill.
In a way, this actually makes them harder to spot from a distance.. until you are right up on them...