Why Hong Kong protesters didn't use their own metro cards
qz.com
qz.com
History shows that all governments fail, but does it show that governments people live in fear of fail faster?
Is history still relevant? The creation of cameras, high speed communications, and computers fundamentally changes the question. The capabilities of citizens, governments, and bystanders have all changed. The capabilities are still rapidly changing, e.g. facial recognition, and drones.
Not using their cards is useless.
It wouldn't be too hard triangulating the cell towers because we have a huge population density and, hence, cell towers attached to nearly every building. But I feel this data would be harder to get hold of and may catch the hundreds of thousands of "innocent" people who live in those buildings.
I don't mean that in a dismissive way: I think that's a seductive thought and the root of many crude, defeatist attitudes towards informational self-determination.