Not a door, it is (probably) an antenna cover.
That is probably the non-conducting cover over the antennas. It has approximatly the right size necessary for the complex multiband antenna (4 band-GSM, 2 band Wifi/BT). It is placed at a good location for getting a reliable link with a low SAR. As the SIM-card is inserted elsewhere, it would suprise me if this is a removable door.
This to me looks like a door for a battery and SIM card. Steve didn't mention anything about the battery NOT being removable...
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That is probably the non-conducting cover over the antennas. It has approximatly the right size necessary for the complex multiband antenna (4 band-GSM, 2 band Wifi/BT). It is placed at a good location for getting a reliable link with a low SAR. As the SIM-card is inserted elsewhere, it would suprise me if this is a removable door.