They're not hiding from us, the mundanes who are so much less powerful that it's a joke. They're hiding from other magical beings.
This could be a mutual masquerade -- the witches and warlocks, after their feud, are both licking their wounds and afraid that being found by the other means annihiliation, and so no mundane can learn about you and start the fatal rumor. Or it could be some less intelligent menace. To be sure, for that, you need a good explanation of why it's not such a menace to mundanes that we would notice.
and you would still have explain why they all comply so well, and what enforcement is necessary, but real danger even to powerful beings is explicable.