TARDIS homing watch

During his exile on Earth, the Second Doctor acquired a TARDIS homing watch.

He later regenerated with this watch, (COMIC: The Night Walkers) and left it on the newly-regenerated Third Doctor's person. (TV: Spearhead from Space)

The Third Doctor found UNIT HQ with his watch, where they had taken the TARDIS. (TV: Spearhead from Space)