When Prof. McGonagall turns into her cat form, she has markings on her face in the shape of her glasses. But what if she wasn't wearing them at the time? Would they still show up after she transformed? If she hung them from her gown, would the mark move from her face to her chest? Or what about when she gets a new pair of glasses that don't resemble her old ones? Would the shape/color of the mark change?
Are an Animagus' markings always the same?
If any of these are true, does that mean she would have to register each possibility/new prescription?