As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AFTER LEVELING OFF AT 18,000 FT, HEAR A LOUD THUMP FOLLOWED BY ILLUMINATION OF RED CABIN DOOR LIGHT. COMPLETED EMERGENCY CHECKLIST. PRESSURIZATION REMAINED AT 6.0 PSID. REQUESTED DESCENT AND RETURN. REMAINDER OF FLIGHT UNEVENTFUL. FOUND PIN BROKEN IN FORWARD UPPER DOOR HOOK.