To get Jennifer into her home, the police take her to the front door of her home. They place her thumb on a small circular reader by the door. Radial LEDs circle underneath her thumb for a moment as it reads. Then a red light above the reader turns off and a green light turns on. The door unlocks and a synthesized voice says, Welcome home, Jennifer!
Similarly to the Thumbdentity, a multifactor authentication would be much more secure. The McFly family is struggling, so you might expect them to have substandard technology, but that the police are using something similar casts that in doubt.