Note: This page is taken from n.wikia.com.
(As written in the N in-game help)
These doors can only be opened by touching an access panel.The locked door is always accompanied by a switch: activating the switch causes the door to open and there is no way to lock it again. Locked doors also block enemies like the normal doors do, until they are unlocked. Usually, it is required to unlock the locked doors to complete a level.