Quote:There must be a sensor somewhere that is triggering the alarm on impact?
My central locking button is permanently lit up!
I've grown to accept that issues like these are part of the classic Porsche ownership experience 
There are no motion sensors that I am aware of on the 968 from factory. What I suspect is your alarm is not arming properly. You close the boot, it arms the car but the action of closing it is either arming it and then the motion triggers either the boot switch or some other switch to re-open and trigger the alarm.
Here is a hypothetical example of what I am talking about.
You open your hood.
Then with the doors closed, boot, glove box, radio etc all in place and switches closed you arm the alarm.
Your door lock light flashes rapidly, pauses then flashes rapidly again, in cycle. This indicates the alarm is on, armed, but a sensor point is not primed (This case it is the hood)
You then go over to close the hood, but un-beknowst to you, when the hood closes, it will open the alarm trigger circuit, which then fully arms and primes the alarm, but due to something worn, or a mis-aligned latch, the hood rubber bump stops push the hood back up ever so slightly, opening the alarm trigger and closing the circuit, now your alarm goes off.
This is how it is supposed to work. I assume your boot problem is the same.
So what you need to do is check two things:
1 With alarm off and doors unlock, is the center console button dark.
2 With everything closed, does the door pull lock knob flash 1 time every few seconds.
If both are true, you are in good shape, if the first shows weirdness, probably an actuator problem. 2nd means you have a trigger point that is not working right with all the places closed.
I had an alarm contact that was zip tied by the previous owner because a clip was lost ($4) This allowed enough movement for it to not always prime and caused strange alarm problems.