Unlike live facial recognition, which scans a camera feed in real time, retrospective facial recognition is run after the fact: an image from a crime scene or investigation is searched against the Police National Database's store of custody images to generate a list of possible matches for an officer to assess.

The specific, long-running controversy here is retention rather than the search technique itself: the PND has been found to hold custody images of a large number of people who were never charged with an offence, let alone convicted, and those images remain searchable in the same way as anyone else's. The Biometrics Commissioner and predecessor oversight bodies have raised this repeatedly as a proportionality problem distinct from the accuracy questions that dominate the live facial recognition debate: even a perfectly accurate search is still searching images that arguably should have been deleted.