None. All alarms are real and are caused by a break in the electrical current passing through the alarm system.
Alarms can be sounded by someone activating a pull station, by something as simple as toast burning near a smoke detector, or by an actual fire.
If an alarm sounds, something caused it. It might not be a fire, but don’t bet your life on it. All alarms should be treated as though they were caused by fire until it can be determined otherwise by a competent authority, such as the responding Fire Department.
Over the years we have developed a dangerous complacency in response to fire alarms due to the overuse of the term “false alarm” to characterize an alarm not caused by actual fire.