And they allow people to walk around them with a labelled route, so you see all the criminals just walk around it.
How many known criminals have biometrics of them in the first place?
Of the criminals that do have identifiable biometrics, do they operate in that area?
We also don't know the false negative rate. Perhaps it's very high in order to keep the false positive rate as low as possible.
These are things pulled out of thin air that could easily explain why 500,000 face scans (not people) provided no arrests/identifications.
The biggest reason probably being as you said... it was clearly marked. Honestly I would be shocked if they found a criminal dumb enough to walk past it, but not dumb enough to have yet to be caught.
A year or two ago there was a "shock horror" piece in one of the far-right extremist tabloids, possibly the Daily Mail, claiming that Glasgow was so lawless "you could get a gram of coke delivered quicker than you could get a pizza delivered".
Shut down by one person's tweet - "yes, because one of them arrives in a Vauxhall Corsa and one arrives in an Audi RS3"
I mean I expect you could get that, for the right sort of money.
Maybe not in Brigton, but certainly in the wealthier parts of the South Side.