panic button
noun /ˈpænɪk bʌtn/
  /ˈpænɪk bʌtn/
Idioms - a button that somebody working in a bank, etc. can press to call for help if they are in danger
Idioms 
press/push the panic button 
- to react in a sudden or extreme way to something unexpected that has frightened you- The prime minister pressed the panic button yesterday as Britain’s economy plunged deeper into crisis.