The release of a dangerous substance occurred, although its origin could not be identified quickly. The operating crews on site, trained in the hazards associated with vinyl chloride monomer, a flammable and carcinogenic substance, initiated the emergency response measures established for such a situation. Despite their search efforts, the faulty valve was closed only 6.5 hours later. An incomplete risk analysis at the design stage had led the operators to fall victim to cognitive tunnelling. This unconscious cognitive bias must be taken into account in procedures and alarm management protocols.