Soon to be outlawed, or are already, in some places. Also
https://en.wikipedia.org/wiki/Gait_analysis and maybe the gadgets you're probably carrying with you, wirelessly spilling some sort of ID into the radio spectrum, easily collectable. Be it by universally deployed broadband routers, over which firmware and functions most users have no control, or gadgets like this, radios in traffic lights, dedicated CCTV in public spaces 'for security', public transit, license plate readers, whatever.