I'm currently building this out as a Raspberry Pi Zero W with an attached camera. The device is powered by a external battery.
I then have a second device that runs a small wifi router connected to a small 4G modem, also with its own battery. The second device needs to be close, but can fit in your bag. Together, the devices provides a small streaming interface, which can run for hours if needed.
All the video is analyzed via the stream on an external machine (I have a desktop box with some GPUs in it). All the analysis and alerting can be run on hardware that you control.
This is a raspberry pi + camera based tool you can "wear" that takes pictures of every "face" you see, and then alerts your phone when you see the same person more than once.View Github repository