Dimensions
At first, Nero thought that the reason he hadn't been able to spot his mother without the help of his dad was because she was moving too fast. But he quickly came to the conclusion that such a thing was impossible.
Though Mystics were much faster than Neophytes, they should not be at the level where they move so fast that they become invisible. Nero could accept that they might only appear as a blur to his eyes, but it was impossible for them to be invisible solely based on physical ability.
That's when he paid greater attention to the other anomalies that became visible as a result of his fathers assistance. The most obvious difference seemed to be that the sky had turned red entirely, compared to its yellow and blue. The landscape also seemed to be more scarce, with lesser vegetation.
Upon closer scrutiny, Nero also noticed that there were fewer animals and insects all around.