Some models don't have rear seat occupancy sensors, so the car knows the seatbelts are unbuckled but it does not know if someone is actually seating in the back or not.
On these models, there's a warning on the dash showing the condition of the rear seatbelts, it can be rid of by pressing OK on the steering wheel, otherwise it will go off by itself after 30 seconds or so.
The rear seatbelts status indicator will also come-up while driving whenever a rear seat occupant buckles or unbuckles a seatbelt.