Perhaps not many criminals use train stations during the day, for example, people probably deliver drugs by bike or car so they don't walk around a busy place stinking of weed.

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.