New Guinea comes to mind as a real life example for a hidden post-hunter-gatherer civilization; from the Wikipedia article on the island:
Before about 1930, European maps showed the highlands as uninhabited forests. When first flown over by aircraft, numerous settlements with agricultural terraces and stockades were observed. The most startling discovery took place on 4 August 1938, when Richard Archbold discovered the Grand Valley of the Baliem River, which had 50,000 yet-undiscovered Stone Age farmers living in orderly villages. The people, known as the Dani, were the last society of its size to make first contact with the rest of the world.
The geography of the island made it specially inaccessible to (European) explorers until the advent of flight.
