Horn blowing awakened all.

After the horn blowing at 2 am, it took me taking the battery cable loose to stop it. Next day I pulled the horn fuse, this was not a real solution.

I found it was an ECU alarm; I have a nanocom, found no error codes logged, looked at the lucas 10AS none logged.

Put the (ECU Alarm) fuse back in and the horn still blows.
This started after a huge clap of lightening, so says the security person, the second time, first it was going off about midnight during the height of this past OTTO hurricane (it shut off by itself); lots of rain but no damage. Then two and a half hours later at a large thunder and lightening clap it was set off again, not to reset. It did not strike near the vehicle.

Any help out there
The truck cranks and runs great, just no ECU security fuse installed. Driven it 90 kilometers that way.

what I would suggest is disconnect battery negative first unplug alarm ecu and immobiliser leave it a good while about 30mins now in your vehicle you will have sensor these detect if anyone is trying to break in to your land rover or is actually in your land rover so they act as shock/motion sensor now find them have mess around lightly tap them whatever your pretty much trying to disturb them sounds silly it's a long story. now once you've waited 30mins connect everything backup as it was your alarm ecu should load on its basic programs anything logged event of someone trying to break into the land rover on your alarm ecu will of drained out as you've allowed the capacitor to fully discharge. something simple its a lot technical jargon but I've had this a lot thunder can cause a sonic boom which can disturb shock/motion sensors fooling the alarm and immobiliser into thinking someone in this case something is breaking into your land rover
only certain software can access your alarm and immobiliser so most don't pick up fault codes
let me know how you get on?

I will do exactly that; lightening discharges here will rattle windows and shake cars. Especially as a hurricane passes over head!
I will report back.
Thank you.
Long time wide area hospital system network tech (retired) so allowing capacitor to full discharge is something I am very aware of.

Wait a minute, I had the battery completely disconnected over night, well at least 5 hrs?

There is no key fob system totally manual key locks. This is a 2002 110 TD5 Costa Rican manual everything and the factory radio has been gone for 8 years; the original 10As and ECU were replaced after a lightening strike 4 meters from the truck, static discharge huge (3 years back) .

in that case then the easiest way to prevent horn from staying on is unplug each plug from alarm ecu one by one until horn goes out. then bell test (testing continuity with multimeter) the wires on that plug with the wires connecting to the horn then cut the wire at the plug. in some cases disconnecting alarm ecu will immobilise vehicle and prevent it from starting. however I can't help but think this will solve the issue as we all know when a alarm is going off we reset the alarm by locking and unlocking vehicle with the fob in this case you don't have a fob so you done the right thing by pulling the alarm ecu fuse. to reset most vehicles I normally just disconnect the negative terminal on the battery wait 15 minutes then connect it back up.

Well, I will pull the Lucas 10AS and open the plastic box to see if it has a burnt spot.
I have used my nanocom and checked all setting no alarms of any-kind are set, visual inspection of circuit board next.

Ok then wire to wire it will be!

Thank you, I'll post the results.