Microsoft researchers have created a mobile device that could forge a trail of “digital bread crumbs.”

The device would collect trail data while the user walked indoors, underground, or in other spaces where GPS signals are unavailable or weak–such as multilevel parking garages that can baffle people who forget where they parked.

A prototype phone called Menlo has an accelerometer to detect movement, a side-mounted compass to determine direction, and a barometric pressure sensor to track changes in altitude.

An app called Greenfield counts a user’s sequence of steps, gauges direction changes, and even calculates how many floors the user has traversed by stairs or an elevator. The app stores the trail data so that a user can later retrace their path precisely.

Greenfield could be used to rescue hikers and mountain climbers, for new kinds of urban street games, or to recover lost items and find friends at a stadium.