a sign or warning of any impending danger, disaster or doom

A 'red flag' is a term used to describe when somebody has a feeling that they are potentially dealing with a manipulative and/or possibly psychotic or sociopathic individual. It's usually used to explain a feeling after the fact.

A person who is unhealthy for one's mental health, always out living their best life messing up everything especially relationships

1. a warning sign

2. subject to a warning

Stopping what you're doing because because something has made the environment unsafe. Derived from the racing flag that is used to stop a race because of unsafe conditions.

a bad sign of what’s to come

1) Traditional symbol of socialist resistance.

2) in an Orwellian reversal, also the song sung by Blair and his cronies at Labour Party Conference