Vehicle including a first substructure and a second substructure arranged such that an interior space is defined by or between the first and second substructures, and an arrangement for determining whether an object is present in the interior space. The arrangement includes at least one ultrasonic transducer arranged on the second substructure and to transmit ultrasonic waves toward the first substructure and receive any waves reflected by objects in the interior space and a processor coupled to the ultrasonic transducer(s) and arranged to determine whether an object is present in the interior space based on reception of waves by the ultrasonic transducer(s). If the vehicle is an automobile and the interior space is the passenger compartment therein, the first substructure can be the passenger seat and the second substructure can be the A-pillar, in which case, the processor determines the presence or absence of a passenger in the passenger seat.