Tracker

A tracker was a device used by the Seventh Doctor. Using it would lead a person back to the TARDIS.

On Armstrong's Colony, the Doctor gave the tracker to Ace and Hex. It was stolen from Ace by Venetia Dalborough. The theft of the tracker caused Hex to experience time shifts. He experienced events out of order which led to the executions of Ace and the Doctor. During one of these events, the Doctor warned him to get the tracker away from Venetia. Hex destroyed it with a rock. (PROSE: These Things Take Time)

When the Fifth Doctor and Nyssa were lost in a field in Kansas in 2158, the Doctor said he wished he'd brought his tracker to find the TARDIS. (AUDIO: The Mutant Phase)