Hi mo, which users are missing?

Be aware that PeopleZen doesn't query your AD *directly*. It enumerates what is in your MOSS user profiles store, which itself may be only partially populated with AD users depending on your user profile import settings.

Oh you said FilterZen in the subject line. The FilterZen User Filter currently lists all user accounts added to the current Site Collection (in the root Web Site's User Information List accessible via 'People & Groups')