You need to check the seat’s wiring harness, as the passenger seat occupancy sensor might be damaged. This can happen if someone frequently presses their knee against the seat; the seat heating system often stops working in such cases as well.
Sometimes,if you have a bag or any object on seats, during bump it creates enough force to trip seat occupancy sensor. In this case, you have to stop and open and close door. My kids discovered this game, they push empty seat and force me to stop and open doors or shut down engine.
I often drive with a small dog in the passenger seat, and if the alarm goes on, I don’t have to stop the car and open the door. The alarm stops on its own once the pressure on the seat is removed.
It’s the same for me in the DS4 and C5 Aircross, so it should be the same in the DS7.
Our Bernese mountain dog of +40 kg does trigger the sensor as well in the backseat. Luckily having 1 seatbelt plugged in is sufficient to silence entire backseat.