Sounds like you have a faulty 'Rotary Latch Module'. This module is responsible for a whole load of weird locking, alarm and window problems. I have had to replace both of mine (one in each door), the first (passenger side) due to my alarm going off at all times of day and night and the 2nd (drivers side, replaced last week) due to the same problem as you. Mine were opening in severe cold weather. As we havent really had any severe cold since I did the fix, I can't be 100% it is fixed.
Replacing is a pretty simple procedure (as long as you're vaguely handy with tools) and you can see a write up I did on the proceedure here:
http://www.mini2.com/forum/first-gen...ml#post3754435 - sorry photos are gone but i'll try ad upload them back to my server if I can find them tomorrow.
Please get more opinions before you go ahead with this as I can't guarantee this will fix 'your' problem. It could also be related to damp in the BC1 (Body Control Unit)