But since such trail data can be retrieved, transmitted to the Internet, and even subpoenaed by the government, this raises the most extreme sort of privacy issues. "How can you control who has access to the data?" Fischbach says. And would employers use it to keep close track of their workers?

The potential applications are numerous. Greenfield could be used for new kinds of urban street games, to recover lost items, to find friends at a stadium, or to rescue hikers and mountain climbers. The researchers cite a 2002 book, Inner Navigation, by engineer Erik Jonnson, who argues that everyone struggles with creating "cognitive maps." Even those who have an excellent sense of direction can be tricked by their own recall, sometimes remembering landscapes in precisely opposite layouts. "I think people have an inner compass," Jonnson says, "and when it goes wrong, the most amazing things happen."

In their test at two different parking garages--one with GPS signals and one without--the Microsoft team started subjects in an adjacent office building and handed each of them a piece of paper listing the color, make, model, and license plate number of a colleague's car. (This kind of problem was familiar to most of the study's participants; one said that losing track of a car in a garage is "catastrophic.") The subjects were given a Menlo device running Greenfield, which had recorded an activity trail, for use in retracing the way back. In some cases, the trail data was enhanced by photographs taken along the route.

Every participant in the study found every car, at least eventually. But since several configurations of bread-crumb data were tested, there was wide variation in how long it took each subject, depending on what kind of information was displayed. Even when they were told what garage floor and quadrant the car was on, subjects often forgot and had to rely on the device for direction.