I've no idea. I'm just an LSL passenger, not an engineer. There is an LED screen above the inside of the door which, when unlit, denotes that the door is locked. When it lights up saying "Door Unlocked" it is then possible to open the door. On the outside of the door, just to the side, another light is affixed to the side of the coach. When this light is lit up amber it means the door is unlocked. When the guard is preparing to dispatch the train, one of his checks is to look down the length of the train and make sure all of those outside lights are unlit. On short platforms when certain coaches are off the platform, the doors in those coaches also get unlocked, so the system seems to be "all or nothing", rather than allowing just certain doors to unlock.
Why not book on a Saphos trip and then you can see for yourself?