false alarm

noun3

1. •
   A warning signal (such as the call of sentry, the ringing of a bell, or the shriek of a siren) which turns out to have been given erroneously.

   • 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. — 2013
2. •
   A thing or occurrence which initially causes fear, distress, etc. but which is subsequently recognized as being no cause for concern.

   • "This may be a false alarm, or a forged letter," said De Bracy. — 1819
3. •
   (no longer used) A person who pretends to be more accomplished or a thing that seems to be of higher quality than is later found to be the case.

   • "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!" — 1911