Hi LutonMan, the way fire alarms activations work is as follows: a person sets off a call point which triggers a sensor to local panel and to master panel. Both panels will show location for investigation by response teams. A feature on a master panel is that a delay can be made for the sounders to activate which allows time for the response team to locate area and from smell and vision determine what has happened and if a fire exists. If the delay times out then bells ring . In the mean time response teams will be positioned to make sure the area is cleared and secured. With no immediate danger to passengers and staff they would have been advised alternative exits as opposed to having passengers straying onto aprons with jet engines running. Had there have been a real fire then rightly all fire exits would have been utilised. Some establishments call it the 4 Cs. Confirm-Clear-Corden and Control. After 20 minutes passengers were let back in the terminal which demonstrates they have highly trained teams!