The aircraft silently circling at 5,000 feet above a metropolitan area can be equipped with sensors to acquire adequate imagery of people on the ground and can then downlink the data to a ground based receiver that can incorporate the facial recognition software for identification. Law enforcement agencies will bang the acquired imagery against their data bases of photos of known or wanted criminals. Of course this data can be placed on the Internet as well.
And the people on the ground are totally oblivious to it.
This is happening today!