[X5M50D]

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Three weeks ago after parking my car on my house drive and locking it, my car alarm siren went off with the horn. Managed to stop the horn from blowing however could not get the alarm siren to stop - tried everything!
After waiting 90 mins (with the alarm blaring) BMW breakdown turned up and the only short term fix they could do was to disconnect the battery. Car was taken to my dealership for repair on the back of a truck.
BMW breakdown suggested that it could be due to a defective battery that's not charging and maybe tripping the alarm?
However BMW technical services confirmed that the overhead control unit needed replacing, with the part finally arriving from USA via Germany on Wednesday. Today I have learnt that this is not the problem and after fitting the new control unit the problem has still not gone away. My dealership have not got a solution to repair and are now awaiting confirmation from BMW technical services what the next step is to rectify the problem.
I'm extremely frustrated that after 3 weeks BMW are no nearer to finding out what the problem is, with my X7 sitting at the dealership compound.
Has anyone had this problem with their SUV and can shed a light to how this can be fixed?
I have been driving around in an X5 loaner but today received an X7 loaner.
I just want my own X7 back