Scanner watch

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, something Sarah Jane could easily cover for. 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. (TV: Invasion of the Bane, Enemy of the Bane)

Functions
The scanner watch could scan for alien life and technology. (TV: Secrets of the Stars, Enemy of the Bane, Prisoner of the Judoon, Mona Lisa's Revenge) Like a geiger counter, it made louder noises as it got closer to an alien source. It could connect to the local power grid and never lost power. (TV: Revenge of the Slitheen)

Sarah Jane detected the Bane inside the Bubble Shock factory and the alien ingredient inside a bottle of Bubble Shock that Kelsey Hooper had. (TV: Invasion of the Bane) It started to beep when the Kalazian Lights were about to appear. (TV: The Lost Boy)

After Maria Jackson moved to America, Sarah Jane used it to scan Maria's old house to see if anything alien might have been left there. (TV: The Day of the Clown) She used it to see if the the Trickster had manifested in 1951. (TV: The Temptation of Sarah Jane Smith)

Sarah Jane also used it to try and determine what was causing the ghosts at Ashen Hill Manor. It indicated that it was neither human nor alien and that it was dormant. (TV: The Eternity Trap)