False alarm

"Then, if they rise, shout the alarm at the top of your voice [...] but be careful not to rouse the camp by a false alarm, for if you do, instead of gaining credit we shall become the jest of the whole force."

Pistorius explained that a security alarm in his house had gone off the previous night, and that he had taken a gun downstairs to investigate what turned out to be a false alarm.

"This may be a false alarm, or a forged letter," said De Bracy.

The terrible announcement that the baby had been taken in the act of putting a doll's frying-pan into his mouth, and was more than suspected of having swallowed a fictitious turkey, glued on a wooden platter! The immense relief of finding this a false alarm!

Cuthbert had often been summoned to her dying bed, only to find that she was alive and well. He expected that this summons would be another false alarm.

Robert Swanson [...] expected civilization to melt down on Jan. 1, 2000 [...] He's not sure yet that Y2K was a false alarm.

"I don't believe you're a bank robber, at all! I don't believe you are even a rustler! You're a false alarm!"

"He's an old false alarm, anyway. I'll bet he never heard a real gun go off!"

"I had a good job, putting in a power plant for his nibs"—he indicated the retreating Gordon with a disrespectful jerk of the thumb—"but I quit [...] Do you think I'd work for this four-flusher if you were in the country?" [...] "You think Gordon is a false alarm?"

"I only came over here to see that darned false alarm of a play of mine put on."