Somewhere in the Pacific ocean is a small island, a few hundred miles off of the coast of the nearest continent. It’s not on any map or sea chart for some reason, and it has no lighthouse or signal beacons, yet there’s enough modern buildings for a hundred or so people on the only navigable beach front on the island. Everywhere else is too rocky to land a boat, and there’s no known airstrip. The island is heavily wooded and large enough to make exploration challenging. Only after the apocalypse do civilians come to live on the island, not knowing why this place is here.
So, what’s hidden? Nothing too grimdark that would result in people sealing the place off or running for their lives, nor should it be an automatic win for a post-apocalyptic community like an infinite source of electricity.