Like planes on radar, their arrival at that point may one day be tracked right to the curb they pause on. Their brief conversation catalogued and kept on a data stream.
In the future, technology may return to the past, where the movements of our clan members was anything but private. At least this is the theory offered up by Oxford University' Nick Bostrom, one of the world's top emerging technology experts.
"In our environment of evolutioary adeptness we lived in small hunter-gatherer tribes in which everyone knew everyone else," says the chairman of the World Transhumanist Association, a group of scientists and thinkers looking for ethical uses of technology to extend human capabilities. "You had a pretty good idea what the others were up to. The same was true for inhabitants of small villages. But with the rise of metropolitan cities, we have got used to privacy and anonymity - ideas that would have been alien to our ancestors."
The English professor foresees technological advances again changing the nature of privacy, as it becomes increasingly easy and cheap to record more and more information on our lives.
"Now imagine that video surveillance, both by law enforcement agencies and individuals, continues to proliferate and is integrated into a network," Bostrom says. "Add to this powerful face-recognition software, and it becomes possible to determine where everybody is at any given time and whom they meet."
However, he adds, it would be essential such a society was two-way - not only the authorities could watch the people, but that people also gain an ability to watch their watchers.
"Even our thoughts might become less private in the future - for example, through better recordkeeping of what we have said and written, and through the development of more effective lie detectors," he predicts.
Hovering cameras which circle city blocks, looking for faces which match national and international watch lists. A new breed of computer that can understand and interpret the raw data on countless lives - separating my wheat from my neighbour's chaff.
Current technology already allows programs to determine sex, age and home region by the sound of a voice. In the future, especially by pinpointing "junk words" - pronouns and prepositions tossed out by the person talking - they will likely become better at finding our lies.
A new breed of camera - for checkpoints, including in airports and at border crossings - is being developed which will study body movement. It would size people up before they've said a word.
On their way into a line, experts foresee people passing by an "artificial nose" in doorways, sniffing for traces of explosives on their scalps and clothing.
Other devices may one day perform an instant analysis of the halo of heat - including odour and skin flakes - that surround every human. Even the shape of our ears may be used to soon identify us.
Using the same concept used by giant radio telescopes to listen to the small energy waves of stars billions of light-years away, a first generation of airport scanners is being developed which would, from a distance, be able to decide if it's a knife or a baloney sandwich in your bag.
Scientists are also working on devices which would use your breath to look for diseases, or as a way to identify you. Sensors in homes may well expand from just security and fire alarms, to transmit DNA-based tests right to a doctor.
Dozens of people in the U.S. have had radio frequency identification (RFID) tags embedded in their skin, to identify them in an emergency. They are likely the messengers of things to come. Their kind promise to keep us safer from disease. Just as they alert our insurance companies of reasons to disallow a policy.
And in the future, sports playing fields may be no less than a computer platform - the turf able to retrace a play or sense whether a player was out of bounds. Could the floors of sensitive buildings be far behind?
Superscanners are being discussed which could detect the stray radiation from computers, allowing officials to know when the man down the street is surfing child porn.
And ask Oregon lawyer Brandon Mayfield about sure ways to track humans. His fingerprints were erroneously matched to those of a suspect in the 2004 Madrid train bombing.
His life - before winning a suit against watchdogs who wrongly brought him down - was not only put on display, it was almost ruined. All because someone read information wrong on the data stream.
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