alarm bell

noun alarm bells

1. figuratively, plural normally A sudden awareness of danger. The specter of women in factories, women in schoolrooms, and women attending — a lot less addressing — public meetings set off alarm bells in the South.
2. Used other than figuratively or idiomatically: see alarm, bell. He put his foot on a concealed button under his desk and rang an alarm bell. He always had two strongarm men lolling around in an office down the passage.