Here is my long shot guess. And I mean guess.
I am thinking that the door loch mechanism on THAT rear door is
bad inside.

There are contacts in it which give feedback to the IDM or BcEM that the mechanical position of the lock is in such a position - such as lock completed.

And that lock mechanism is broken to a degree whereas the switch does not move in conjunction with the mechanical mechanism or moves but sends back the wrong signal.

A few tests:

It would be interesting to see what happens if you press your key fob
button twice to put the Discovery into Superlock.
I bet it will not try to unlock from that mode.
But who knows..


Also Drowsap is spot on.

If you can pull off the rear door panel then unplug the harness from the door latch mechanism and see how things go.

I am betting the door lock mechanism is bad.

 

The contact signals the BCU by grounding. The black ground wire appears in C0805 pin 4 connector in the door pillar, and again at C0552 pin 10 (ground header) in the console. The BCU monitors all the doors except drivers door as a common "zone". So a bad switch on any of the non driver doors would cause a mislock, etc., or an alarm later on. I suspect more than the door alarm contact is involved. It may be easier to get at door pillar than console or door panel.

Keep in mind that Rover provided all this at considerable expense, and was unable to budget for an oil pressure gauge....

 

Here's the update. Voltage at battery was 12.5 when the phantom came a calling. The voltage drop (around 3/10 of a volt) occurred when the phantom unlocked the disco. That's the same drop when the unlock button is pressed on the fob.
The phantom unlocked when it was superlocked... and when the alarm was armed.

So now some info that can hopefully lead to a solution. With any door left open the phantom disappears for some time but eventually comes back and starts unlocking. Everytime the door locks unlock, the interior lights come on, and the exterior indicator lights flash...just like pressing the unlock on the fob.

It's like it's a nighttime thing. Is there some photo cell on these beautiful animals???

I'll owe you all a beer. The only caveat would be having to travel to northern Michigan to collect!!

 

Why don't you unplug the wireless receiver and see if phantom (O ghost who drips) comes back?

 

Ah, I know what it is...
An aftermarket alarm is installed in the truck and it is unlocking it.

A forgotten alarm.
Previous owner.

Look for rats nest wiring somewhere...
Or tell tale RED wires.

Most stuff like this is people who have tinkered with the wiring and it has gone stray.

Look at the fuse boxes for a wire jammed into a fuse by a copper blade or
those wire hack things that add a wire to a circuit via a little square hunk of plasic and matal whereas you use pliers to push down a wedge into the existing wire and the new wire to the alarm.

Has to be some of this nasty stuff hiding.

I am thankful my 2001 is boring and unmolested.

 

I will. Didn't know what it would do to the alarm.

It's located in the headliner forward of the rear sunroof correct? A phantom living up there would explain the sagging and sometimes wet headliner...

 

Here is pix of receiver location. Your choice on unplug it there, or remove other trim to get at wire somewhere else. The grounds from it run to a header connector in the luggage area, may be even easier to deal with.

If you have aftermarket alarm, good luck and be sure to have the RAVE and the install guide for the alarm. Alarm installerz go by the ADT code of worst practices; always darn trouble with any dang thing all the damn time.

 

I've looked for aftermarket alarm wiring in all the fuse boxes with no luck.

 

Pg 18/152 of the Electrical diagrams shows header 707. I don't see any of those grounds going to the receiver. What am I missing?

 

Page 22 of 126 in the circuits manual for the 2000 D2 (another manual for later D2) shows receiver, with two earth wires (black) going to header C0707 on pins 7 & 2. Page attached.

On the 2003 up schematic you are correct, no grounds at that point.

If receiver is going wonky that would be one thing. Once you get into swapping out electronic modules (like BCM) price jumps up quickly.