I've actually been playing in my mind with different consequences of a different spell, so here it is, in very raw form:

She had, at first, ascribed it to a momentary premonitionary sort of coincidence, but the occurrences kept piling up: she knew when a car was coming around the bend; she had known when Anne had tried to throw the ball at the back of her head in PE; she could tell Nellie's arm wasn't broken; and in the end, she realised she could focus on this newfound ability and *use it*. It was, of course, Edmund who finally put two and two together, just like he had all those years ago - well, a year ago - in the ruins of Cair Paravel, concerning the trouble with time.
"Suppose it's the spell you've read and now you can see things other people can't see in this world?" he said now.

(I was once struck by the fact it says "A spell to make hidden things visible" - that's much broader terms than just "invisible", isn't it?)