What was found
(CAN) AT LAST FLIGHT OF THE DAY, PILOT NOTICED PRESSURE VERY LOW ON REAR CABIN HAND FIRE EXTINGUISHER AND REPORTED SNAG. UPON REMOVAL, MAINTENANCE PERSONNEL FOUND UNIT VERY LIGHT, AND PRESSURE ZERO. THE Q.A.CHECKED: BOTTLE LOOSELY UNSCREWED FROM HEAD. NOTE: SEAL OF HANDLE: NOT BROKEN. PILOT SAID HE CHECKED IT OK ON HIS PREFLIGHT. EVIDENTLY UNIT HAS BEEN TEMPERED WITH. RECOMMENDATION: THAT THE NEXT OF THE BOTTLE BE DRILLED FOR SAFETY WIRING TO THE HEAD.