false alarm

A benign situation initially mistaken for an emergency. Mary thought she was going into labor, but it turned out to be a false alarm. When the smoke detectors started beeping, we thought the building was on fire, but it was just a false alarm caused by faulty wiring. Whenever the fire department comes to shut off a false alarm, you can always count on a few looky-loos hanging around the fire trucks.

a false aˈlarm

a warning of something, especially something unpleasant or dangerous, which does not in fact happen: They thought the packet contained a bomb but it was a false alarm.