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.
(..) - "Minkey"? - What? - You said "minkey." - That is correct, yes. Chimpanzee monkey. So I let them both off with a warning. The beggar was the lookout man for the gang. - That is impossible. - Why? He was blind. How can (..)1975 The Return of the Pink Panther
(..) And because I expect to be transferred... back to the detective division at any moment, I will let you off this time with just a warning. Thank you, monsieur l'agent. But you must get yourself a proper license. - First thing tomorrow. - Try to do something about your filthy monkey. Oh, one moment. (..)1975 The Return of the Pink Panther