Every grocery store self checkout has cameras on the whole area as you walk up, then cameras on your face, and then pretty good self identification via your payment card/rewards.
Hypothetically speaking couldn't you collect it from any security camera in an area where you have positive ID on people? Airport security comes to mind.
With all the trade in personal data in the US I assume it's only a matter of time before places like grocery stores start selling it.
Why stop at just gait though? Geometric fingerprinting of various body parts is also possible. Palm geometry readers have been commonplace for a long time.
That's just one that comes to mind. Anywhere that has positive ID would work. Which if we account for data brokers can potentially include anywhere you use a credit card. (Yes that will not be a perfect 1 to 1. Obviously that doesn't really matter in practice. LE is used to dealing with lots of noise.)
But suppose there's no hit. That's a hit in and of itself. Someone just needs to have the idea to have the software that ICE agents use flag anyone who fails all recognition methods (facial, gate, etc) because it means they haven't been through an airport, haven't crossed a border in a legal manner, don't have a passport, and don't show up with any of the data brokers.
