Issuing something to non-citizens only obviously does not work.

What would stop any non-citizens from simply not producing that something when controlled and stating to the controlling police-officers: "I am a citizen"

This discussion somehow reminds me of the time before we got our machine-readable id cards here - lots of panic that big brother will watch us permanenty, and most people did not notice that the most important part of the old id cards (little booklets) was already machine-readable - its number. With that number it is very easy to look up all the other data ...