A Secrecy Sensor is a type of Dark Detector. It is shaped like an ariel, but it is gold in colour and very squiggly. It vibrates when it detects concealment and lies.
The Fake Alastor Moody had one in his office during Harry Potter's fourth year at Hogwarts School of Witchcraft and Wizardry. However, it was no use to him at the school because it was constantly humming, as (supposedly) students were always nearby teling lies about why they had not completed their homework assignments. However, it is more likely the result of him masquerading as another.
When Harry first visits the Ministry of Magic in August 1995 the wizard at the security desk checks him over a Secrecy Sensor before letting him enter further into the Minstry.