qui vive
noun /ˌkiː ˈviːv/
  /ˌkiː ˈviːv/
Word Originlate 16th cent.: from French, literally ‘(long) live who?’, i.e. ‘on whose side are you?’, used as a sentry's challenge.
Idioms Idioms 
on the qui vive 
- paying close attention to a situation, in case something happens
- He's always on the qui vive for a business opportunity.