Is there any difference between ‘wondering whether that hadn’t been Cedric’s plan’ and ‘wondering whether that had been Cedric’s plan’?

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. For example:

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.

Solution 1:

The negation implies more hesitancy in the speculation (effectively here, a lesser degree of likelihood).

People quite commonly say things like I wonder if it's not better to [adopt my suggestion]. The negation there could be seen as more 'polite', in that it emphasises the speculative native of the statement.

Apart from politeness, the implied hesitancy may make it easier for a hearer to consider objectively what's being suggested, rather than immediately recognise and react to what is in effect a challenge to his existing thinking.