As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
"DURING LEVEL OFF FROM CLIMB TO 7500', PILOT WAS IN THE PROCESS OF LEVELING OFF WHEN THE "DOOR OPEN" ANNUNCIATION CAME ON. WE LOOKED AT THE FRONT CANOPY LATCH AND FOUND IT CLOSED. I (THE INSTRUCTOR) TOOK CONTROL OF THE PLANE WHILE MY STUDENT TWISTED AROUND TO LOOK AT THE REAR DOOR LATCH. AT THAT TIME THE DOOR OPENED VERY FAST". THE AFT PASSENGER DOOR EXITED THE AIRCRAFT OVER A HEAVILY WOODED AREA SOUTH/EAST OF BFI. AIRCRAFT LANDED WITHOUT INCIDENT.