If you get very close to the dampers at the top of the ventilation window, you may feel a weak flow of air.
A Danish Technological Institute study of residential housing units with ventilation windows in Højstrupparken (Copenhagen) confirms that, over a period of nine months in two test apartments, no draughts were measured coming from the windows.
During a storm or other extreme weather conditions that cause high wind pressure on the building façade, the window closes automatically via a storm vent at the bottom of the frame. The storm vent counteracts the stronger flow of air through the window and eliminates any draught thus caused.