The transporter security system was a system on Deep Space 9 that identified potentially hazardous objects before they could be transported.
In 2373, the system failed to detect a remat detonator hidden somewhere on Trentin Fala's person, despite being programmed to do so. This lead Jadzia Dax to speculate that whoever planted the device on understood the space station's security protocols well. (DS9: "The Darkness and the Light")