I don’t think she’s being unreasonable, though. Whoever is doing this is presumably as powerful or more powerful than the demon king, which significantly narrows down the candidates.The most suspicious looking ones are usually not the culprits and are red herrings.
Girl forgot to look at the other possibilities that it may not be any of them, and perhaps an outsider or a traitor from her side.
I’m more surprised that she concluded the culprit must be a living person, and not some sort of phantom or spirit.