What was found
CABIN ALTITUDE EICAS WARNING (CABIN WOULD NOT HOLD PRESSURIZATION). CABIN AUTO INOPERATIVE EICAS CAUTION MESSAGE WITH INTERMITTENT MASTER AURAL WITH NO ASSOCIATED DISCRETE LIGHT EICAS ALERT. AIR TURN BACK TO DEPARTURE STATION. EMERGENCY DECLARATION WITH NO ADDITIONAL PRECAUTIONARY ACTIONS. AIRCRAFT GROUNDED FOR UNSCHEDULED MAINTENANCE ON ARRIVAL. POST FLIGHT, LINE MAINTENANCE IDENTIFED MAIN DECK CARGO DOOR VENT DOORS OPEN. ENSURED SECURITY AND INTEGRITY OF MDCD VENT DOOR IAW B767 AMM 52-30-00. CONDITIONAL INSPECTION AND CABIN PRESSURE LEAK CHECK GOOD IAW B767 AMM 05-51-24 AND 21-31-00. NO FAULTS FOUND. AIRCRAFT RELEASED TO SERVICE.