I'm sure that was a lie about a test. In tests, we always pre-announce. But we'd never test a place of public assembly at noon, where people could get hurt if they exited, or to use the proper emergency exits is violating security rules - causing a complete re-search of all pax, and hours of delays. Try 3 AM. Plus, you would have heard a sequence of alarms, as the test crew moved along the fire alarm zones.

More likely a malicious false alarm, like in high school or college.