|Muggles' Guide to Harry Potter - Magic|
|Features||Detects dark magic|
|First Appearance||Harry Potter and the Half-Blood Prince|
The Secrecy Sensor is a device that detects certain classes of Dark magic.
Hermione tells us that the Secrecy Sensor can detect "jinxes, curses, and concealment charms", and so are useful for detecting a reasonably wide range of Dark magic. They can be operated by the non-magical; the first time we see one, it is being wielded by Filch, who is a Squib. Hermione later says that they are useless to detect love potions, because love potions are not Dark magic; and they can be concealed by putting them in perfume bottles, no Concealment charm is necessary.
The limitations of Secrecy Sensors seem to be that they can only detect those sorts of magic that are associated with secrecy. Hermione mentions that simply putting love potions in perfume bottles, which disguises their nature, is enough to get them into the school, and that secrecy sensors won't trip because there is no curse, jinx, or concealment charm.
Hermione suggests that the cursed necklace that Katie Bell is carrying in that book would not pass the Secrecy Sensors, but does not have a chance to explain why not. If her statement is accurate, then we must assume that the Sensor would read the existence of the concealed curse on the necklace.
Secrecy Sensors, and their limitations, are an illustration of the way the author thinks through the magical world that she has created. We will learn later of the existence of Probity Probes, another way of defeating concealment. If it was possible to reliably detect Dark influence, or the actions of Dark wizards, the story would be significantly weakened; if there is no challenge, there is no story. The author understands that a writer, creating a fictional universe, must give the universe a coherent set of rules, and must keep everything within that rule set; it is unfair to the reader to suddenly unveil a skill that effortlessly wins the battle for the protagonist. So the limitations on the Secrecy Sensor, making it possible to evade its detection, make it more possible to believe in its existence as a part of the Wizarding tool set.
While we don't know when the mead that poisoned Ron came into the school, Professor Slughorn suggests that it had been intended as a Christmas present for Professor Dumbledore. Filch, as we see at the initial mention of the Secrecy Sensors, is already using them in October, and we suspect will not stop using them between then and Christmas, as their introduction seems to be in reaction to Voldemort's return, rather than being an arbitrary decision of Filch's. It is unknown whether their use extends to the faculty, but it is possible, especially if the decision came from Dumbledore. It is possible that the mead was scanned by one of Filch's Secrecy Sensors as it entered the school, and was passed as clean. Non-magical poison in a mead bottle would be as unlikely to trigger a Sensor as love potion in a perfume bottle, of course, so even if the necklace mentioned above had been found, the mead would not.