A proximity system has its own issues,
Indeed, that sounds likely. Not that I have personal experience.
In the context of a column like this and its anecdotes, one of those issues is that most of the stories come from an entirely different era. Computers have had screen lock for as long as they've had screens, and inactivity-based logout for as long as I can remember, but any more sophisticated measure of proximity is surely an altogether different story.
 OK, that wouldn't've looked like modern screen lock, but clearing the screen then refusing any input that isn't an accepted unlock serves the same purpose.