What is the purpose of that door, hatch, whatever?
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Looking at Airbus's diagram gallery on the A380, this image suggests that the panel is to store and deploy the emergency evacuation slides.
This is also supported by another picture from the same document, in which similar doors are seen under each second row entrance.
On the bottom deck the slide is packed inside the doors, but on the top deck the are in a small compartment underneath the door.