Your smoke alarm may sound when its very cold outside, or if a door adjacent to a heated area is opened, like in an entryway. This is due to condensation (water vapor) in the detection chamber. The sensor is a particle sensing device, so when water condenses in the sensor, the unit will go into alarm. The operating temperature of a smoke alarm is between 40 and 100 degrees Fahrenheit  (4-38 degrees Celsius). If the unit is in a garage or attic, you should consider replacing it with a heat alarm, which is not affected by condensation.