When you have your vehicle's ventilation system turned off and set to non-recirculating mode, but you still feel air coming through the vents, there are a few possible explanations:
Ventilation system design: In some vehicles, even when the ventilation system is turned off, a small amount of air may still pass through the vents due to the design of the system. This airflow is usually minimal and intended to provide some level of fresh air circulation in the cabin.
External air movement: Air can enter the cabin through natural airflow caused by external factors such as driving at high speeds, wind, or air pressure differentials. These external forces can create air movement that enters the cabin through small gaps around doors, windows, or other openings.
HVAC system malfunction: There might be an issue with the vehicle's HVAC system, such as a faulty blower motor, control unit, or a broken blend door. These malfunctions can cause unintended airflow through the vents even when the system is turned off. If you suspect a malfunction, it's best to have the HVAC system inspected and serviced by a qualified technician.
It's worth noting that some vehicles have an "auto" or "defrost" mode that automatically activates the ventilation system to prevent fogging on the windshield. In such cases, the system may override your manual settings and introduce airflow through the vents.
If you find the airflow bothersome or suspect a malfunction, consulting a certified mechanic or contacting your vehicle's manufacturer for guidance would be a recommended course of action. They can provide specific information based on the make, model, and design of your vehicle.