That's a tough one. I can't even begin to think why the boxes may have stopped the alarm going off.
Maybe there's an electrical plug with a dodgy connection that the boxes are pressing on ? - that's the best I can come up with at the moment.
Long may it continue though! I'll report back if/when it goes off again. Very strange! When I got the garage to hook it up to a computer, the fault was a faulty boot something or other. So, then i replaced the boot lock and so on, and it was still going off!