Looking at the outside view in the video in Reply #27, those appear to be the two hinge pins sticking out horizontally. The front one is clean - minus the big guide retaining (travel limiting) washer and locknuts. The rear one appears to still have something still attached to it - the remains of the hinge guide ripped from the door?
One of long bits at the bottom is probably the spring that carries the weight of the door so it can be delatched..
Looking at the Boeing video, there was a spring on both hinge pins, I can't see anything to show that they are captive though.
When the door/plug opened it would have immediately been hit by a 400MPH sideways blast. The movement on the front pin would have been a bit more linear (pivoting on the rear one) so the hinge guide probably hit the end of the front pin with a lot of force. That might have been enough to knock off the washer and locknuts. With the front pin disengaged, the twisting motion on the rear pin would probably have been enough to rip the rear hinge guide off the door, leaving the apparent remnants still attached to it.
Now that they've found the door (in somebody's yard according to the BBC) there ought to be plenty of telltale evidence. I suppose it's always possible that, if the door had been removed for access, the hinge pin end washers and locknuts might have been left off as well as the locking bolts. A examination of the front hinge pin end with answer that one, it will either have snapped or stripped threads, or intact ones.
In terms of whether the door should have stayed with the plane, that's a tricky one. On the one hand, it coming off could easily have damaged the rear flight surfaces. On the other hand having a non aerodynamic door crashing up and down in the air blast might have had a tin-opener effect on the bottom of the airframe via the hinge pin mounts. It's not as if anyone was going to be able to close it again. I'd be surprise if there isn't distortion around the hinge pin mounts on the airframe anyway.
P.S. Looking at the BBC report again, the decompression was enough to rip open the cockpit door and suck out the co-pilots clipboard and headset. If that happened at 16000ft, it doesn't sound good in terms of helping the pilots retain control during a sudden depressurization even. Surely the (anti-hijack?) door ought to be strong enough to let the cockpit depressurize in a slower and more controlled fashion.
https://www.bbc.co.uk/news/world-us-canada-67909417(There's also further indication that the plane has shown decompression warnings on previous flights and had been withdrawn from routes over water! [Ed: as tom66 indicated])
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