When you're on the qui vive, you're especially vigilant, carefully keeping watch. Your dog might believe it's her responsibility to be on the qui vive all day, barking at every little noise she hears.
You'll almost always find this term in the phrase "on the qui vive." It comes from the question French guards and sentries would commonly ask visitors to a castle or fort: "Qui voulez-vous qui vive?" Literally, this means "Whom do you wish to live?" — in other words, "Whose side are you on?" Today, being on the qui vive means that you're on the alert, like a sentry, watchman, or guard dog.