Bedrockgames
I post in the voice of Christopher Walken
Much like how finding a painting in a haunted house may not mean that the painting is supernatural, but it introduces the possibility.
The possibility existed before it was found. There is a reality that exists in the game outside what the players are hearing from the GM. And once found, that painting is either supernatural or it isn't. It doesn't become supernatural the moment its nature was revealed to the party (unless the GM is ad libbing everything). The idea that everything is fuzzy and in flux until the players see it or encounter it, to me runs very counter to how a typical sandbox and living world GM would think of things in the setting. Much of the point is to create a sense of a real concrete world outside the player characters. And a key way of doing that is trying to be consistent with these kinds of details. Now the parts can move around (an NPC doesn't have to be in the same room waiting for the party to get there: this is actually one of the main examples used in "Living Adventure" discussions: the NPCs are alive and move around with their own goals, like PCs). But the point is the GM is meant to take things like NPC motives and goals, very seriously when deciding what that NPC is doing, and where it is (not simply decide arbitrarily that the NPC is in location Z because that is convenient for some other goal----like having a plot or something)