Dead battery following alarm/electrical oddities

Hi all!

I'll preface this post by just being clear from the get go that I'm only now really trying to get into the more hands-on part of car ownership. Despite being 35, until now I've always been a straight-to-the-garage kinda guy. However, I purchased a 2005 MG TF in August and now doing the upkeep myself. 

That out of the way, to the issue at hand and I apologise for the length of the post, but I like to be concise. Saturday evening, I got in my car at the same time as I always do every Saturday to do a 77 mile drive from my sons house back to mine. I unlocked the car, got in and started her and then proceeded to do the full drive with no stops. When I got home though, that's when the weirdness started. I got my things and closed the driver side door and as I pressed the lock button on the fob, I heard the horn been once and no sign of the usual alarm arming noises - weird, never heard that before. I was tired so left it as it was and continued the evening as planned.

The next day, Sunday, I knew it was time to get working on it. My first instinct was to assume it was the car warning me that a door or similar was open, so I reopened the driver side door and closed it again before checking the passenger door as well. Again, the car beeped on lock. I unlocked it once more and tried all four possibilities, the rear boot, both doors and the front bonnet, all of which closed firmly like they always do.

With the car still beeping once on lock, I assumed the issue was still not resolved and so I started to do some Googling and, as expected, found myself directed here towards a couple of related threads, but they were unfortunately unhelpful.

I decided that maybe it was something power related, so with little effort, I removed the black cable from the battery for a few minutes to let the system drain out before reconnecting it. Upon reconnecting, I started the engine and despite everything being closed, I noticed a new light that had never been illuminated before, the door open warning light. Back to Google and results suggested that one of the latches wasn't latching properly, so I opened everything and using a screwdriver, was able to latch both doors and the rear boot lid, with the interior lights and the boot light all going out, suggesting that the car was satisfied that they were properly closed. I couldn't work out how to 'fool' the front bonnet though, but with it closed properly and normally, I put the key in again and started once more to find that the door open light was still on.

With it getting late and dark, I was getting fed up with the problem and new it was something I needed to look into another day, and living behind a locked gate, I was comfortable leaving the car unlocked for the time-being. I drove to the Tesco Express near me, and there were no issues with the drive or the starting/running of the car. Got home, didn't lock it and that was that.

Fast forward two days later to today, Tuesday, and I was getting something out of the boot when I decided to just have a little look to see if the car was still being silly. I put my key in and turned it, expecting to see the red 'door open' light, but was instead greeted with nothing. No lights, no mechanical sounds, nothing. The battery has as far as I can tell gone completely flat.

As I don't have a battery charger on hand, I got a friend to come over and we used some jump leads to try and get some juice in to hopefully start it. I wasn't expecting it to work out, and those expectations were met. After a short while, still nothing. I've seen a couple of threads on here regarding sudden dead batteries, and followed their guidance on things like battery negative cable corrosion etc, but I think it's fairly obvious that the dead battery is linked to this random door open alarm.

So I can be sure I've listed my checks off, this is what I've done/looked at/checked; Have confirmed that all interior lights and rear boot interior light go off when the respective doors/lid are closed, so it's not that. I know the soft top isn't alarmed/powered in anyway but have unlatched and relatched that anyway for good measure. I have checked all the looms I can see and they all look to be in excellent condition, with nothing visibly damaged or looking like it needs to be replaced. I have checked all the relays under the front bonnet, and despite some oxidisation, nothing has blown. Checked all fuses under the steering column and they all look ok.

I'm at a loss on what could cause this door open thing to suddenly trip, and I am 100% certain that there was no red warning light on the drive home, so this has happened as I've reversed on to my driveway and gotten out of my car. 

 Firstly there are no such thing as alarm oddities..... the damned things are a consistent source of problems, as there hasn't been one made yet that isn't a total piece of rubbish.
Having said that, your car probably has the later Pektron system which isn't anywhere near as bad as the earlier Lucas 5AS system is at causing flat batteries.

I don't think the door latching would've been my first port a call, I would suspect the door switches themselves, or the wiring loom that runs up to the boot by the boot hinge, this is a first stop check for a lot strange electrical behavior concerning our cars.

Alarms and immobiliser systems can play up for various strange reasons, when I bought my old Ford Falcon ute, it wasn't a runner and had been sitting for @ 2 1/2 yrs, t he plumber that owned it couldn't start it no matter what he tried, so he offered it up for sale, I went around to have a look and noticed that the interior light in the cab wasn't working, I immediately made him a low offer.... which he accepted out of frustration. I then replaced the dirty interior light fuse and the ute started first turn of the key....leaving, the
plumber had a very sick look on his face.
Now I knew how Fords Smartlock immobiliser system works, it uses the door light switches to tell it if the doors are open and if there is something  is outside of any of it's parameters it won't let the engine start! I have since bypassed the Smartlock nonsense as it is a constant cause of woes, and not worth the pain and suffering.... you know when they feel the need to call something smart......it isn't!
The whole concept of  an immobiliser that is tripped by something as silly as a dodgy interior light is utterly stupid!

Your problem will be as equally silly.