From Longman Dictionary of Contemporary Englishfalse alarmˌfalse aˈlarm noun [countable] MISTAKEa situation in which people wrongly think that something bad is going to happen Fire fighters responded to a false alarm at one of the college dormitories. The patient was okay – it was a false alarm.
Examples from the Corpus
false alarm• Your apprehension is based on the fear that the change might be a false alarm.• But the patient was okay, it was a false alarm, please thank them and send them on their way.• Those genes would produce a harmless protein that, once inside the body, would sound the false alarm of infection. 