The detectors are most certainly cheap ion mobility spectrometers. They ionize the sample and sort the resulting cloud by the speed at which the ions move in an electric field in a low pressure gas. The resulting spectrum is fairly unique to each chemical.

The obvious problem is that you get a superposition of spectrums of multiple chemicals from the sample and that the devices being cheap and fast produce "blurry" spectrums, so you get a lot of false positives.