Funnily I was woken up and about 1.30 this morning as mine was going off. Couldn't see any reason for it but it was chucking it down. Perhaps it set mine off? Was fine for the rest of the night.
Yeah this happens to me all the time as well. There is one thing you could try is to press the button on the drivers door which disables the movement sensor which you would normal use on a ferry. Hope this helps.
Mine went off the other day too, it must of been the hail as i was looking out the window at the hail when it went off. It wasn't coming down that hard if I'm honest.