Alarm continuously on / Fuse 67

[Katerin]

Hello!
I am new to this forum. We own a 2001 M3 E46. We're located in Spain.
I come to you since we are desperated after many checks without success. I cound not find any thread with this matter, so I create a new post.
The problem is that one day, when arriving to the garage to go for a drive with she, a high pitched and continuous sound was coming out of the car. The same noise of an alarm but continuous, not a beating one.
It happenned suddenly, without having made any mod or work in the car. One day we parked it in the garage and some days later it was "shouting". We do not know for how long it could have been that way, since we live about 15 minutes drive from the garage and go there every 1-2 weeks.
We though this scream should be the alarm, and checked for the fuses. Removed fuse 67 (which looked fine, by the way) and it stopped sounding. We replaced it by a new one, and the car started shouting again.

As per the fusecard, number 67 applies to alarm system, immobilizer and centre-mirror. Important to remark that our M3 does not have an immobilizer for a long time now (4 years roughly), which is previous to this happening, so this system was discarded.
With regard to the central mirror (only has the "red nose" and the darking option when light comes from cars behind; no rain, whipper or any other sensors), we disconnected its connector and plugged the fuse 67 again, and there it came the alarm immediately. So as of now, mirror got discarded. Should we had an additional mirror we would have tried to connect this to discard it with more guarantee.

So right now we are running without the said fuse number 67. But we would like to solve the matter so that the related systems work as they should.
I don't know if it could be related, but the remote openning/closing does not work since we removed the said fuse except from very time to time. Just in case this fact could help...
By the way, we replaced the battery and the alternator 2 years ago, and voltage is perfect. Whilst this poltergeist is happening for few months. And the hood-open switch is unabled on purpose too. In any case, the alarm would sound in beating mode (normal mode) and not as a continuous beep.
Did anyone have the same or similar happening? Any ideas what could be or anything else we could check?

Thanks a lot in advance for your time and help, and apologies for the long text (but I wanted to explain every detail and check we made).
Hope you can help us sort this out.
Kate

[Amos]

Do you have access to any type of scanner, PA Soft or INPA?

I would start with scanning the alarm module for error codes.

[Slideways]

Yes, scan the General Module for error codes using INPA. That should point you in the right direction. Otherwise, you'll have to check the alarm components, one by one, with good working units from another car to narrow down the problem. The hood switch is the easiest to check and a common failure. Then you have to check the tilt sensor in the trunk, the ultrasonic sensor in the headliner and the alarm siren itself.

[thegenius46m]

Hood sensor is 90% of the time the issue. If unplugging it with the fuse reinstalled stops it from going off, then that is likely the culprit.

[Slideways]

I think OP updated another forum post that a faulty siren was the cause. After unplugging it, the alarm stopped going off randomly. I've seen this problem on another make/model.

[thegenius46m]

That doesn't sound like the issue. Unplugging the siren won't stop the lights from flashing. It'll just stop the alarm from making noise. Read the codes in the gm5 and stop guessing. That will tell you the real issue.