When should I stop ordinary self-checks for this pattern? The safest answer starts by preserving timing, location and trigger information rather than assigning a cause from one clue.
What this question is really testing
fans, stack effect, wind and duct imbalance move air through both intended and unintended routes. The observation becomes more useful when it can be compared with insufficient outdoor air, duct imbalance, envelope leakage, blocked transfer path and room-specific heat gain or loss.
A useful next observation
Log exhaust fan, heating and cooling cycles. Record the result together with conditions that stayed unchanged so the comparison remains honest.
Safety and handoff boundary
Combustion backdraft concerns, carbon monoxide alarms, strong fuel odor or unsafe electrical equipment require immediate safety action rather than airflow experiments. Share room-use conditions, fan states, door behavior, supply and return observations, temperature differences and any combustion appliances nearby.
