What was found
CRUISE AT FL360, ACFT EXPERIENCED A REAR BAY DOOR MWS INDICATION IMMEDIATELY FOLLOWED BY CABIN ALTITUDE. CREW & PAXS DONNED 02 MASKS. CREW DESCENDED TO SAFE ALTITUDE & RETURNED TO BASE. INSPECTION REVEALED TRASH ON PLUNGER FOR TOILET PREVENTING PROPER SEAL. INSPECTION ALSO REVEALED DONUT & CAP WERE NOT PROPERLY INSTALLED AFTER LAVATORY SERVICE, NO PART MALFUNCTION NOTED. PRESSURIZATION FORCED THE LAVATORY DONUT & CAP TO UNSEAT. THIS CAUSED THEM TO CONTACT LAVATORY SERVICE DOOR WHICH ILLUMINATED THE REAR BAY MWS LIGHT. DONUT & CAP UNSEATED ALSO VENTED CABIN TO ATMOSPHERE CAUSING DEPRESSURIZATION. FLUSHED LAVATORY SYS, PROPERLY INSTALLED DONUT & CAP, ACFT PRESSURIZED NORMALLY ON DECK IAW MM. ACFT FT TEST FLOWN AT FL 360 FOR 14 MINUTES & PRESSURIZATION WAS NORMAL.