In most cases, unless you witness the fire, you won’t know the reason behind the alarm.
The alarm’s purpose is to warn you that you are now in danger.
Sometimes they have a fault, sometimes it’s a false positive. But it’s just as likely that they have detected smoke and a fire is now feeding on everything it can reach.
So it’s in your best interests to calmly leave the building until the Fire and Rescue services tell you that it’s safe to return.