A Secrecy Sensor is a device used to detect concealment and lies.
From the Story
A Secrecy Sensor is a Dark detector, and looks much like an extra-squiggly, golden television aerial. It vibrates when it detects concealment and lies.
Discovered in Harry Potter and the Goblet of Fire
Professor Moody has a Secrecy Sensor in his office at Hogwarts. He tells Harry that it is no use within the school, as there is too much interference due to students lying about why they haven't done their homework and other fibs. The Sensor has been humming ever since Moody arrived at Hogwarts.