Why did the villain tear apart the city in pursuit of the MacGuffin?
Why did the vase fall over and shatter with no one in the room?
Who was that advising our Hero? He called himself George, but the only George who knew those secrets died seven years ago.
Is his survival from a freak accident really a sign of favor, and an indication that he's the chosen one?
Is her running into him with the information he needed really just a coincidence?
Did the villain really just happen to run into that location, so that his crime happened to reveal a much greater danger, or did he stage it in desperation?
Sometimes, of course, you get an answer to that in the course of the novel. Indeed, if it's the focus of the hero's efforts, it takes serious skill to have the hero, having spent the story searching for the info, conclude at the end that no power manifested by the MacGuffin would do anything he can imagine the villain wanted. But for the cat burglar to go vaulting over the roofs -- the hero can't give chase because he must contain the greater evil -- to stop and look back and smile, leaving the hero to ponder whether it was his escape or the discovery the burglar was pleased with, can be quite satisfying. (Besides, while it limits the possibilities of dramatic irony, it deepens your identification with the POV character.) Having some but not all the supernatural pranks turn out to be tricks can leaving a sense of mystery that can deepen the novel and make it seem more like reality, where not everything is cleared up. Having a character declare this can not be coincidence may seem a contrivance to paper over its manifest coincidence, but even that can be better than just leaving it there, and as part of a conflict of worldviews, it can be a moment of contrast.
Some readers don't like it even in small doses. I have gotten critiques complaining that a character's acts in a scene were inexplicable, even when the POV clearly doesn't know what's going on himself. But sometimes it can be pulled off on a large scale, and even in a manner crucial to a novel.
What works do you know that don't clarify everything? Does it work? Does it actively add something to the work?