I don't recall my i-MiEV alarm ever going off, so I have zero experience with it. PV1 addresses the issue, with the suspicion that the system was already 'armed' when the 12v battery disconnect was attempted.
This discussion left me a little perplexed, and my question remains: if disconnecting the 12v battery does not silence the alarm, where does the power to drive the horn and flash the lights come from? Seems a little far-fetched for the dc-dc converter to get activated by a triggering of the alarm system.
Thinking about this further, did merely loosening a battery terminal cause an intermittent connection which triggered the alarm which subsequently scared the bejeezus out of NippyCar and the full disconnect was never made? I mean, the horn is right there in your face if your head is under the open hood (bonnet)!
First-world issues... gone are the days when most of the time I not only left my Saab unlocked but kept the keys in the ignition as well, figuring it would be a rare thief who would not only know how to start it but also know how to use the weird column-mounted shift.