Time path indicator

The time path indicator, or time path detector, was a component of a TARDIS used to detect time machines in the Time Vortex which were on the same flight path. It could also be removed from the console and used to detect time machines.

The time path indicator in the First Doctor's TARDIS signalled its crew that the Daleks were following them in the Dalek time machine following their escape from Aridius. (TV: The Chase) It later detected the Monk's TARDIS following them. (TV: The Daleks' Master Plan)

The time path indicator also registered when it detected the Master's TARDIS near the Darkheart system. (PROSE: The Dark Path)

The Second Doctor later removed the time path indicator from the console and put it in a cupboard, claiming it wasn't very useful. Zoe Heriot later found it and reattached it to the console. Unfortunately, this resulted in Jamie and Zoe becoming aware of a past TARDIS present in the vortex, but they were unaware that TARDISes were automatically programmed to avoid such accidents anyway, which resulted in the two TARDISes colliding and crashing on the planet Urbinia six months apart. This created an alternate timeline where the Daleks were able to complete the Time Destructor, nearly causing a devastating temporal storm, before the Second Doctor was able to avert the original collision. Once the timeline was restored, the Doctor found himself having removed the indicator again. Zoe reminded him he said it was making an annoying noise. (AUDIO: Daughter of the Gods)

The Seventh Doctor, having received a warning about the Timewyrm, removed the time path indicator from the TARDIS so he could detect its coming. (PROSE: Timewyrm: Genesys) He later used it to track her down after she escaped. (PROSE: Timewyrm: Exodus)

Behind the scenes
In The Chase, the component is called the time path detector, but is renamed the time-path indicator in the novelisation and all subsequent stories.

It first appeared in The Chase but disappeared from the console in following serials. Daughter of the Gods offers an explanation.