EzekielRaiden
Follower of the Way
Exactly.Except that you're the DM. Contriving any sort of connection as to why the NPC knows something is ludicrously easy. And, you can simply rely back on the old saw of "it's a logical consequence" and be magically right every time.
It's simple. The DM is the one eyed man in the land of the blind. Even further than that, every single thing in the game world can only be viewed through the lens that the DM presents. How the DM describes something will always affect the reactions of the players. It's completely unavoidable. So, the NPC knows that the party has been captured? How? Well, let me roll out my magic typewriter and cobble together a couple of paragraphs explaining how that happened. It's all post hoc and complete hogwash, but, it's all part of a "living world".
The person who controls all of the contents of the world, and who can change those contents if/when/as they feel like it, and who contains ALL ability to ever observe any portion thereof, can always change those contents to be as they wish them to be.
Nobody can pull off perfect deception instantly time after time with no preparation. Nobody's that good.
But that isn't what GMs have. They have--as has been indicated and agreed to by multiple posters--months of time to figure out a way to explain something. Many, many sessions to plant seeds, to...gently tweak things, to reveal that what was thought to be true before was merely a superficial appearance, and the real truth is something else, and much harder to reach. And they can respond to any complaint with, "Hmm, that's pretty mysterious, isn't it! You'll just have to trust me that there's a good explanation", and then spend the next three weeks figuring out a good explanation.
One brain vs five brains, the five brains will (eventually) win every time, if they have equal thinking-time. But they don't have equal thinking time, and the five brains aren't actively collaborating all the time either. The one brain can spend a hundred times as much time thinking as the five brains do--and, often, the one brain is much more experienced at that kind of thinking. Plus, the task isn't the same; asymmetrical conflict often means one side has an easier burden than the other. Detecting a deception isn't the same as crafting one, and, in general, detection is harder than crafting even without the massive advantages conferred by the GM who has absolute declarative power.
Not only that, but even if you suspect something, oftentimes it will be so far along, it may not be worth following up, right? It's not like people have infinitely long memories, nor that a single mysterious event remains supremely relevant for 100% of a campaign's run. If the GM only needs to do this now and then, rather than all the time, the odds tip even more in the GM's favor that they can pull this off.
Or, for those who prefer a more pithy presentation:
"Why didn't you tell me? You told me Vader betrayed and murdered my father."
"Your father...was seduced by the Dark Side of the Force. He ceased to be Anakin Skywalker, and became Darth Vader. When that happened, the good man who was your father was destroyed. So what I told you was true...from a certain point of view."
"From a certain point of view?!"