In a chemical plant, a phosgene (COCl2) detector tripped while the phosgenation unit was undergoing a rinsing phase with monochlorobenzene (MCB) as part of a manufacturing shift process. The structure was evacuated and the distillation supply line turned off. As a precautionary step, 2 subcontracted employees whose ‘phosgene’ badges had become coloured from exposure (concentration < 5 ppm/min) were examined at the plant infirmary. Since the alert only concerned a single very sensitive detector (with a 0-0.5 ppm scale) and only lasted a few minutes, washing operations resumed. A second phosgene detection then appeared on the same detector, 5 min after resuming supply to one of the distillation columns. Like the first time, the measurement proved to be low (0.4 ppm) and of limited duration. The site's internal emergency plan was nonetheless activated and all operations in the affected unit halted. Fire-fighters dispatched from the chemical platform secured the perimeter and performed atmospheric measurements, which returned negative results. The emergency plan was lifted 30 min after activation. The quantity of COCl2 discharged was estimated at 16 g (2 times 8 g), based on atmospheric dispersion calculations. Given the small size of the leak creating the incident, identification of the exact emission point was complicated and took several days. Ultimately, a corrosion hole on the connection circuit between the unit's vacuum system and the containment device was exposed. The detector that first triggered the alert was located just a few meters away on the same floor, downwind of the pipe. Pipe operations had remained normal, with a slight pressure drop of a few millibar. The simultaneous presence of a major gas flow and the corrosion hole explained how a small quantity of phosgene could be discharged. Moreover, during the investigation, other anomalies were revealed, i.e. defective column feed flow meter, porosity of a nitrogen supply hose on a column, etc. Various remedial actions were implemented: replacement of the defective flow meter, replacement of the installation's nitrogen hoses by rigid pipes, replacement of the pipe section where the incident originated, and improvement of monitoring and operational systems on one of the distillation columns.