1. A system for detecting the presence of particular contents of a vehicular compartment, comprising: a detector system arranged to generate an output signal indicative of the absence of an object in the compartment when an object is not situated in the compartment and a different output signal indicative of a particular object in the compartment when such an object is present in the compartment whereby different output signals are generated for different objects, said detector system comprising receiver means for receiving waves generated or modified by the object in the compartment when present such that in the absence of an object, the output signal generated by said detector system is indicative of the absence of an object and when an object is present, the output signal generated by said detector system is representative of the waves received by said receiver means; and processor means coupled to said receiver means for processing the output signal generated by said detector system when an object is present in the compartment, and representative of the waves received by said receiver means, to create a signal characteristic of the object in the compartment which is indicative of whether the compartment is occupied by a human occupant or empty, an inanimate object in the compartment, which is not part of the vehicle, being treated as if not present, whereby this system is coupled to a reactive system for affecting a component in the vehicle when the created signal is indicative of a human occupant.