A detector is a machine that responds to particular substances in a consistent way. Everyone who flies on an airplane must pass through a metal detector, which beeps when it senses metal.
A "lie detector" works by measuring pulse and eye movements as a person responds to a series of questions. A radioactivity detector looks for high levels of radioactivity in nuclear power plants. Since the invention of detectors, the idea of more fantastical detectors has filled the human imagination. A love detector: why not?