What was found
(CAN) WHILE CLIMING THROUGH 23,000 FT. THEY EXPERIENCED A RAPID DEPRESSURIZATION, LOSS OF CABIN PRESSURE (O2 MASKS WERE DONNED BY FLT CREW). CREW MANUALLY CLOSED OUTFLOW VALVE HOWEVER AIRCRAFT WOULD NOT PRESSURIZE. CREW REQUESTED DESCENT AND RETURN TO BASE - NO EMERGENCY WAS DECLARED. AIRCRAFT LANDED WITHOUT INCIDENT. MAINTENANCE INSPECTION REVEALED THE UPPER BEACON ASSEMBLY LIGHT MISSING - NO DAMAGE IN HOLE OR AIRFRAME. ASSEMBLY REPLACED AND AIRCRAFT GROUND CHECKED SERVICEABLE. AIRCRAFT RETURNED TO SERVICE WITH NO FURTHER INCIDENTS TO REPORT. NOTE: PART HISTORY SHOWS UPPER ANTI COLLISION LIGHT ASSEMBLY REPLACED APRIL 17/03 DURING 'C1-3 CHECK' AT 3RD PARTY VENDOR PER NON ROUTINE FORM.