This sensor is a type of Dark Detector that is golden in color and resembles a squiggly television aerial. It vibrates when it detects concealment and lies (GF20).
The fake Moody had one in his office when Harry visited the first time. He said it's no use at Hogwarts because of too much interference from students lying about doing their homework, causing it to hum all the time (GF20).
Secrecy Sensors were among the Dark Detectors carried on shelves in the Room of Requirement during D.A. meetings during Harry's fifth year (OP18).
Filch searched all students upon entering the Entrance Hall for the first time during Harry's sixth year, which led to the confiscation of a shrunken head owned by Crabbe (HBP11). He also used one on incoming and outgoing students on Hogsmeade weekends that year (HBP12). Ron was jabbed a few extra times by the long, thin instrument after asking aloud why Filch was using it on outgoing students.
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Notes
Secrecy Sensors are not to be confused with Probity Probes (OP7, HBP6, DH26), which are described as a long thin golden rods. The difference in function between the two is unclear, as they can both be used actively to search individuals. One difference between the two may be in how they are operated: we see Filch use a Secrecy Sensor in a jabbing motion, while Probity Probes are passed up and down the front and back of the searched individual (though one was said to be stuck up someone's "...", indicating that this difference may be user-specific) .