> couldn't you collect it from any security camera in an area where you have positive ID on people? Airport security comes to mind

Going out on a limb and guessing illegal migrants aren’t going through airport security.

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.

Plenty of people overstay their visa. And then we have the administration changing the rules to make people that were legal, suddenly not.