A thing about the world is this: some dungeons are subtle. They open up their portals somewhere inconspicuous, kidnap people slowly at the maximum range they can, and aim to go undetected for as long as possible, maybe even getting to sneak away into the night before the coordinated local arm of humanity can find them and try to take them down.
(This makes them sound weak, but subtle dungeons come in all sizes and classes. Many newly spawned dungeons try to be subtle. But so does Nightmare, and it's much better at it.)