Falling prices may be a 'warning sign' of a recession. [=may indicate that a recession is coming]
There were storm 'warnings' [=warnings that a storm was approaching] issued for the area.
I'm giving you 'fair warning' that you had better not discuss politics at dinner tonight.
(..) and said it was destroyed almost as soon as your father sailed away. But that they allowed some of the settlers to escape, so they would take the word to your father, as a warning. He never mentioned it. Ask him now. We don't know where Ragnar is. He left soon after his defeat in Paris. (..)2016 TvShow: Vikings Title: The Last Ship Season: 4 Episode 10