You know what!? Completely oddly, after 7 years, my 156 has started this too.
1. Press the key to remote central lock
2. Get 3 beeps
3. After 1-2 seconds, get a final long beep
4. After anything from 15 seconds to 2-3 minutes, the alarm goes off.
The alarm will stop, then wait and go off again in cycles.
Pressing the key does stop the alarm and unlock the car.
I did notice whilst driving along that on my dash console, the door indicator kept flickering with the boot lit up, but it was defo closed.
Does the alarm use the open/close sensors in addition to other stuff? If it could be the rear boot open sensor, where is it located? I ask as the actual mechanism etc seems fairly clean and everything that moves seems to move ok.
Any advice appreciated!
PS: sorry for tagging on here but it seemed the right thing rather than a new related thread