So None Of The Humans In Gravity Falls Noticed When Ford Was Replaced By His Twin, Which Makes Sense,

So none of the humans in Gravity Falls noticed when Ford was replaced by his twin, which makes sense, he was a recluse in the woods who probably only infrequently came into town for coffee or groceries. And also a secret mind-wiping cult was on the loose.

But as reclusive as he was around the humans, he got VERY social with the local supernatural entities. He raised a shapeshifter, he kept a gnome in a cage, he yelled at Steve with a megaphone. The local paranormal community was probably very familiar with the excited six-fingered human who traipsed around the woods conducting interviews and taking notes.

So, what did they think when he suddenly vanished, to be replaced by a five-fingered con artist who completely ignored the supernatural entities and began bringing humans around the shack?

I propose this:

None of the supernatural entities realized that he's Ford's twin.

They assume that as part of Ford's studies, he summoned up his own evil doppelgänger, and it killed him and took over his life. As doppelgängers are wont to do.

Which makes Stan a member of the paranormal community in their eyes; but if he'd rather try to blend in among the humans, hey, that's his business. They just won't send him invites to the town's magical meet-ups. He is, after all, an EVIL doppelgänger.