How did Agatha know of Wanda's (supposed) death?

When we have the Agnes of Westiview portion on episode one, it's all about Wanda's death: she finds her corpse, she finds that the Darkhold and all its copies have been destroyed, etc. But it all plays out inside her head. So she would have to (at least subconsciously) know what happened to Wanda and the book in order to have this murder mystery delusion. Yes, Rio was eventually there, but Agatha found the corpse by herself and she went to the library by herself, so buried inside her she must've had the knowledge.

Now I'm wondering whether she "found out" because of her connection to the Darkhold (being a former user), to her connection with the Scarlet Witch (she's trapped in her spell) or if someone mentioned it to her prior to the beginning of episode 1. Or if it's just a plot hole (I really hope it's not).

Does anyone have a clue? Did I miss something when watching that explains it?