It seems the NYPDs argument is that releasing information on the vans will allow people to predict where they are (or more likely be told ala speed cameras) and so avoid them (civilians as well as criminals).

That's kind of the point of a deterrent.

You cannot scan every car on every journey, at least not without a massive spike in cancers, so this is a deterrent.

And it's a secret deterrant....

So it won't catch anyone because you can't scan everyone, and it won't deter anyone because no one knows it's there

Seems a waste