Ooo now there's an interesting question. To be an RF detector, it would need to measure the induced current (or, similarly, the induced time-varying magnetic field) in a conducting part of the detector (aka antenna), preferably with the size of the antenna matched to the wavelength of the RF field you are trying to detect.
Whereas a metal detector generates its own EM field, and measures the perturbations on that field caused by the presence of conducting (e.g. metallic) objects.
Does that clarify anything for you
