Transmat:Technology
The scanner watch was a device used by Sarah Jane Smith to detect alien life, and identify the species and its origin. It gave off a humming sound when in use. Superficially, it appeared to be a normal wristwatch, but the face of the watch concealed a scanner screen. Like her sonic lipstick, it was left by the Tenth Doctor for Sarah Jane inside K9.
We like tracking our technology round these parts. Only problem is that the show is about a time travelling scientist. And he's spawned shows like Torchwood that are entirely focussed on alien tech. So there's a lot of tech sittin' around the joint. Be bold, though, and start your journey at category:technology.