A Zygon detector (PROSE: The Day of the Doctor [+]Loading...["The Day of the Doctor (novelisation)"]) was a machine that the Tenth Doctor used to detect Zygon DNA.
Besides lighting up when in the presence of a Zygon, the machine could microwave frozen dinners from up to 20 feet and download comics from the future.
The Doctor thought the machine identified Elizabeth I as a Zygon, but it had in fact detected that her horse was one. (TV: The Day of the Doctor [+]Loading...["The Day of the Doctor (TV story)"])
Behind the scenes
It resembled the Timey-Wimey Detector from Blink in many ways, being made of a bunch of random objects, making a "ding" sound, and being able to remotely prepare food.
The Tenth Doctor also had a similar implement in Planet of the Dead.