London’s Metropolitan Police Service is conducting a two-day trial of face recognition this week around several locations in the city.

What’s happening: This is the seventh public test by the force, with three more set to take place before the end of 2018. The Met is yet to disclose details for the next pilots but says it’s considering football matches, music festivals, and transport hubs as settings.

Why? The police say it’s being used to identify wanted criminals from its “watch list” database. The system only retains faces that match those on the list, which are kept for 30 days, and all other data is deleted, according to the Met. The NeoFace technology they’re using is made by Japanese IT multinational NEC.

Controversial: The tech has already become quietly pervasive in the US, but it’s still a relative novelty in the UK, and not a particularly welcome one in certain quarters. Privacy watchdog Big Brother Watch has filed a legal challenge against police use of face recognition, warning that it’s being used without legal backing or sufficient public knowledge.

False positives: It’s not clear whether the tech even works: 98% of the face recognition matches in a previous Met Police trial turned out to be incorrect. It’s even less accurate for people who aren’t white or male.