To me that makes familiars very valuable. An animal capable of seeing through any mundane disguise in relation to their masters appearance is crazy good. I could see it being common practice for paranoid merchant princes to have a pet bird to validate their identity if that is how they worked. Possibly almost expected in some circles as an authentication method. While this could be seen as needed to protect the sanctity of the familiar merit it does have an impact on the world and does screw over those interested in disguise.

If the familiar had abilities that explicity interacted...