I work at a dealer so I've seen a few alarm related problems. The first and most likely (although not related to anything you're mentioning regarding temperature) is that the remote button pads are having problems, the rubber inside expands and that causes the alarm to activate by itself. This is now a known issue at all Volvo dealerships and easily, quickly fixed.
If that's not your problem, then the next possibilities are that either a interior mass movement or inclination sensor (one of two sensors) might be acting up. We had a car last spring that'd act up intermittantly and wasn't easy to diagnose, fix. I had to take the car home for a few days just to duplicate it because it wasn't acting up at work. After it'd sit in my driveway for about 2 hours, the alarm would go off all by itself. It turned out to be the inclination sensor (senses if a tow truck is tilting your car). The sensors can be temporarily disconnected electronically through the vadis cart (Volvos dealer PC) to test them and leave them off until a new sensor arrives.
If that's more like what you're experiencing, then I'd advise your dealer to call Volvo tech hot line for assistance if they can't solve it otherwise. Unless there's fault codes indicating a problem, doing that trial and error method can take time. Our customer wasn't very patient or happy over the experience as it took many days even though he had a free loaner (buried us in CSI) but we did fix his alarm.