A weapons scanner was a scanning device attached to a door or any specific section of a passageway. It could be used as a way to ensure that dangerous objects are not taken past a certain point. They were specifically seen to be used on Cardassian military installations, such as the space station Terok Nor.
In 2346, while still under Cardassian control, Terok Nor was equipped with many of these devices in areas where VIPs needed extra protection from possible hostile actions. (DS9: "Wrongs Darker Than Death or Night")
In 2369, Miles O'Brien used the weapons scanner on one of the docking ports as a way of disabling one of the Hunters that had boarded Deep Space 9. The device was shown to be extremely effective at blocking passage through the port while any type of powered weapon was being worn, and the power setting could be ramped up to a point where the discharge from the device could knock a person unconscious. (DS9: "Captive Pursuit")
The station's first visitor from the Gamma Quadrant, a member of the Tosk species, was alarmed when a phaser being worn by Chief Miles O'Brien activated the weapon detectors in one of the airlocks in 2369. O'Brien later rigged the weapon detectors to stun a Hunter who was pursuing the Tosk. (DS9: "Captive Pursuit")
Later the same year Neela, a Bajoran member of the station's engineering crew, disabled the weapon detectors so that she could take a phaser onto the promenade and stage a failed attempt to assassinate Vedek Bareil Antos. (DS9: "In the Hands of the Prophets")
The Bajoran officer who tried to assassinate Commander Benjamin Sisko in 2369 told him about his report of the weapons scanner shortly before he tried to use his assassination device. (DS9: "Dramatis Personae")