It can expose contaminated contents and hazards beyond an ordinary domestic appliance check. The system needs appropriate specialist procedures.
What the observation means
A quiet tank is not evidence that it is empty or safe. Electrical controls and confined spaces can add risks.
A useful next step
Keep the cover secured and stop contributing wastewater if the system has failed. Record external alarms and identifiers only. Do not enter, lean into or attempt to drain a tank yourself; arrange the appropriate qualified response for that equipment.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.