Bold of you to assume they'll continue to listen after the keyword "fire". In the worst case, they'll actively doubt you and, worse, start doing so in a very public way ("they're just claiming the fire is out").

And yes this shit has happened in the past. Panicked people are uncontrollable and in a critical situation, priority #1 is to avoid panic to rise at all costs because panicked people can turn a critical situation into outright disaster.