I’m thinking a whether clause with a negative sentence means the speaker thinks the situation is less likely. However, I can’t find any explanation in dictionaries at hand.
He snapped it shut again, worried that the sound would attract Filch, wondering whether that hadn’t been Cedric’s plan – and then, making him jump so badly that he dropped the egg, which clattered away across the bathroom floor, someone spoke. (Harry Potter 4 [US Version]: p.461)[Bold font is mine]
N.B.: ‘It’ is a magical egg which is wailing loudly while being left open. Filch likes punishing students.
Why did the writer use a negative sentence in the whether clause? I’m wondering if it has some nuance in it.