Dumbledore's Army
In "Order of the Phoenix", Secrecy Sensors are used at the Atrium Desk in the Ministry of Magic upon visitors to the government locale. Later in the book, Harry mentions to Dumbledore's Army that they can be easily fooled like their other dark-detecting counterparts. In "Half-Blood Prince", due to Hogwarts' new stringent security measures, Argus Filch is assigned to inspect every student entering the castle with Secrecy Sensors. All the owls flying into Hogwarts are also placed under this measure to ensure that no Dark object enters the castle through mail. Hermione later explains that although Secrecy Sensors detect jinxes, curses, and Concealment Charms, they cannot detect love potions, as they are not considered Dark.