alarm bell

(figuratively, chiefly in the plural) A sudden awareness of danger.

Example: Used other than as an idiom: see alarm,‎ bell.
2000, James Hadley Chase, An Ear to the Ground
  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.
1998, Michele Gillespie, Catherine Clinton, Taking off the white gloves: Southern women and women historians
  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.
2010, John M Findley, Just Lucky
  I only got a glimpse of him, but alarm bells went off. He wasn't rushing over to give me a pat on the back for stopping to help.
2011 October 1, Tom Fordyce, “Rugby World Cup 2011: England 16-12 Scotland”[1], BBC Sport:
  England were shipping penalties at an alarming rate - five in the first 15 minutes alone - and with Wilkinson missing three long-distance pots of his own in the first 20 minutes, the alarm bells began to ring for Martin Johnson's men.