Ventilation can be part of the system's intended operation. Blocking it may create another problem while concealing the original smell.
What the observation means
Different units use different vent or filter arrangements. An opening that looks unnecessary should not be capped on appearance alone.
A useful next step
Follow model-specific owner maintenance only. Report persistent odour with any leak or drainage change. Do not fit a generic air valve, tape over openings or attach an improvised hose as a cure before the installation and appliance have been assessed.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.