BTP say [0] that "the images and biometric data of those who don’t cause an alert are automatically and immediately deleted." - if that's true, they shouldn't be able to tell that it's the same faces being scanned multiple times so it has to be 500k total scans rather than, say, 500k unique faces across 2 million scans

[0] https://www.btp.police.uk/police-forces/british-transport-po...