It's certainly known that they do the tracking part - in the UK as long as a decades ago you'd see police at demonstrations with cameras catching as many faces

It's certainly known that they do the tracking part - in the UK as long as a decades ago you'd see police at demonstrations with cameras catching as many faces as possible (and weren't secretive about their purposes). What they then did with that information could range on a scale from nothing to conspiracy theory level, I'm not sure.