While the door in and of itself might not be in a pressurized area - if it were to come loose, could it have ripped the skin of the aircraft, and could that rip have extended into a pressurized area? Alternately, if the door came off entirely, could it have hit and damaged another part of the aircraft?
Yes, it could, but I'm not aware of any precedent, on any aircraft type. Also, top of climb is not when you'd expect it to happen - peak TAS was earlier, in the climb.