Google Now is constantly gaining new abilities that are generally awesome, if a little bit creepy. One such feature, brought to our attention today, is the ability to keep track of flight prices.
This is another automatic feature whereby Google infers your intention and presents useful info on that basis. In this case, if you are eyeing a flight or itinerary through Google Flights (it does not appear that this works with other travel booking sites right now), Google will make a note of that and drop a helpful card into your Google Now screen to let you know when the price of that flight changes.
As with most Google Now features, we can't exactly be sure how widely the "flight price monitor card" has rolled out, or even exactly how to trigger it, but shopping around on Google Flights seems to be a good starting point. Let us know if the card has shown up for you.