An overseer.
Short version - a lock with facial recognition abilities.
Long version - overseers are a form of magical technology, in layman's terms, a knot of magical substance that can be charged with various forms of magical commands. Overseers are widely used across the city, in locks, cameras, just about everywhere. Their problem is that they require magical maintenance, which rises their overall cost. In other words, the hardware is cheap as dirt, but the programming is costly.