It uses a drop sensor.
So it only shows sold out if you actually try to vend an empty column and nothing comes out. It will detect that no can or bottle has fallen and will allow another selection or a refund, and mark that column sold out.
If you select an empty selection and nothing comes out and it takes you money and does not mark it sold out you may need to recalibrate the drop sensor, or replace it if calibration does not work.