We had a big storm today in the south of the UK with a lot of rain. I drove on very wet roads (perhaps driving through deep water
couldn't do that in my M3). once parked up the boot opened when I locked the car (via comfort access), I would press the button on the boot lid to close, but it kept re-opening. Probably happened about 5 times. I forced it closed and used the lock button on my key and that kept it shut.
I had previously had a phantom boot open when the car was in a petrol station and I was refuelling, again very wet roads.
My key on both occasions was in a pocket in my jacket so could not have been pressed, there seems to me a link between significant water under the car and the kick sensor thinking a foot is under the car.