I think I may know what the problem is.
I have a 2 week old MKX which did the exact same thing to me. Last Friday, after washing my car, I left it parked in front so it can sparkle in the sun. I went inside to cleanup the rest of the house and take a break. About an hour later I hear a car alarm going off outside and it goes on for about 5 minutes, I said to myself "Jeez! this idiot needs to go turn off his car alarm, I can't even watch tv!...", so I go to take a looksy outside to see who this poor fellow is and it turns out to be my own car
I run outside and try disarming it but my key didn't work. Finally after holding my panic button on the key for a bit, it disabled the alarm. First thing that popped in my head was whether or not someone was trying to steal the car. So I circled around and didn't notice anything out of the ordinary. After a few minutes of standing there stratching my head, it was time for me to go pickup my kid at the sitters.
I start up the car, then I noticed the message screen showing "rear right passenger door ajar". This probably explains why the alarm went off because the door was opened. So I go outside to check, but door seems to already been shut tight, so I just open/close it again. Get back in the vehicle but the message is still there. I get back out again and open/close the door a couple of more times, but the message doesn't go away. It was getting late so I brushed the problem aside and went pickup the kid.
On the way back, I stopped to fill up with gas, now because the "message" was still there, I couldn't check any other Info statuses at all on my info screen. I tried the reset button but it doesn't work either. So I'm standing out the gas station slamming my rear door about 20 times over. By this time my kid is screaming his lungs out from all the noise and onlookers are thinking I'm abusing him.
So I hop back in the car and drive back home still wondering what the heck is going on with this car. Halfway home, I stop at a stop sign and next thing I know, the door alert light turns off and the message goes away, WTF?! I was hoping Ashton Kutcher would jump out somewhere and scream "you just been punked!!!" but that surely didn't happen.
My theory is due to the hot weather messing up the door sensor, it triggered the alarm to go off. So in your case, because of the cold weather its possible your door sensor was out of wack too and this triggered your alarm. The dealership couldn't find any problems and they don't have any reports or even a clue to what happened. I'm betting there will be more reported issues regarding car alarms going off for no reason and this may hopefully lead to some type of fix or recall.