A method of operating a radar sensor system for detecting an occupancy in an interior of a vehicle, with vital sign monitoring; the radar sensor system comprising - a radar transmitting unit being configured for transmitting radar waves towards a scene within the vehicle interior, - a radar receiving unit being configured for receiving radar waves that have been transmitted by the radar transmitting unit and have been reflected by an object within the scene, and - a signal processing and control unit that is at least configured to derive range information, Doppler information and angle-of-arrival information from the received radar waves; and the method including at least the following steps, which are to be carried out in an iterative manner: - operating the radar transmitting unit for transmitting a radar wave towards a scene within the vehicle interior, - operating the radar receiving unit for receiving at least one radar wave that has been generated by reflection of the transmitted radar wave, - operating the signal processing and control unit for o decomposing the received at least one radar wave into range, Doppler and angular information, and o detecting and monitoring vital signs of occupants (28, 30) in each region of interest (38, 40, 42) of a plurality of predefined different regions of interest (38, 40, 42), characterized in that the step of operating the signal processing and control unit includes o quantifying and tracking a movement in each region of interest (38, 40, 42) of the plurality of predefined different regions of interest (38, 40, 42) by angular gating and range gating, and o for each region of interest (38, 40, 42), determining whether quantified and tracked movements in the scene are related to an occupant (28, 30) or to external or internal disturbances, based on a fulfillment of at least one predefined condition concerning the quantified and tracked movements and the detected and monitored vital signs, wherein the at least one predefined condition includes that if strong movement is present in all or several range regions of interest (38, 40, 42), a strong disturbance is likely to be present, and if movement is detected only in a region with a seating position, and vital signs are found, it is likely that the seat is occupied. - a radar transmitting unit being configured for transmitting radar waves towards a scene within the vehicle interior, - a radar receiving unit being configured for receiving radar waves that have been transmitted by the radar transmitting unit and have been reflected by an object within the scene, and - a signal processing and control unit that is at least configured to derive range information, Doppler information and angle-of-arrival information from the received radar waves; - operating the radar transmitting unit for transmitting a radar wave towards a scene within the vehicle interior, - operating the radar receiving unit for receiving at least one radar wave that has been generated by reflection of the transmitted radar wave, - operating the signal processing and control unit for o decomposing the received at least one radar wave into range, Doppler and angular information, and o detecting and monitoring vital signs of occupants (28, 30) in each region of interest (38, 40, 42) of a plurality of predefined different regions of interest (38, 40, 42), characterized in that the step of operating the signal processing and control unit includes o decomposing the received at least one radar wave into range, Doppler and angular information, and o detecting and monitoring vital signs of occupants (28, 30) in each region of interest (38, 40, 42) of a plurality of predefined different regions of interest (38, 40, 42), o quantifying and tracking a movement in each region of interest (38, 40, 42) of the plurality of predefined different regions of interest (38, 40, 42) by angular gating and range gating, and o for each region of interest (38, 40, 42), determining whether quantified and tracked movements in the scene are related to an occupant (28, 30) or to external or internal disturbances, based on a fulfillment of at least one predefined condition concerning the quantified and tracked movements and the detected and monitored vital signs, wherein the at least one predefined condition includes that if strong movement is present in all or several range regions of interest (38, 40, 42), a strong disturbance is likely to be present, and if movement is detected only in a region with a seating position, and vital signs are found, it is likely that the seat is occupied.