I think large public buildings like airports and railway stations tend to have a fire alarm system that works on a "two hit" principle: a single smoke detector or "break-glass" point being activated will notify staff who will then make urgent enquiries to see what, if anything is amiss. The alarm itself will only be activated automatically if two independent "stimuli" (e.g. from a break-glass point *and* a smoke or heat detector) are received.

I'm sure that the immigration halls at airports have been designed with this eventuality in mind so that they can maintain control of the border even in the event of an emergency evacuation: the expedient may well be as simple as the emergency exits directing people airside rather than landside.