false alarm

noun
1.
a false report of a fire in progress to a fire department.
2.
something that excites unfounded alarm or expectation: Rumors of an impending transit strike proved to be a false alarm.

Origin:
1570–80

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
false alarm
 
n
1.  a needless alarm given in error or with intent to deceive
2.  an occasion on which danger is perceived but fails to materialize

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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American Heritage
Idioms & Phrases

false alarm

A warning signal that is groundless, made either by mistake or as a deliberate deception. For example, The rumor that we were all going to get fired was just a false alarm, or Setting off a false alarm is a criminal offense. This expression, first recorded in 1579, today is often used for a report of a nonexistent fire.

The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
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Example sentences
Instead, when nearby monkeys dropped their food and ran, the one that had
  sounded the false alarm moved in to scoop up the bounty.
Perhaps at that point, law enforcement personnel might have had reason to
  believe that this was a false alarm.
If the home-loving citizen would sit down and think about it, he would realize
  that this is a false alarm.
They were afraid, they believed they were in danger, though maybe it was all a
  false alarm.
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