You see crimes being committed and instead of calling the authorities, you continue to collect data (license plate numbers, phone calls, conversations).
It might have only become clear to her at a later point that it's actually criminal rather than just unethical - if she was able to tell from the beginning it is illegal, she would probably be a lawyer or a paralegal, rather than a nanny.
I don't think you copy down the license plates of cars in the driveway just because. In my opinion, she knew exactly what she was doing and what this information could mean to other people. I think this is straight up blackmail.