warning signal vs signal

warning signal and signal both are nouns.

Word	Noun	Adjective	Verb	Adverb
warning signal	Yes	No	No	No
signal	Yes	Yes	Yes	No

As nouns, signal is a hypernym of warning signal; that is, signal is a word with a broader meaning than warning signal:

Other hypernyms of warning signal include sign, signaling, signalling.

warning signal (noun)	signal (noun)
an automatic signal (usually a sound) warning of danger	any nonverbal action or gesture that encodes a message
	any incitement to action
	an electric quantity (voltage or current or field strength) whose modulation represents coded information about the source from which it comes

warning signal (adjective)	signal (adjective)
	notably out of the ordinary

warning signal (verb)	signal (verb)
	communicate silently and non-verbally by signals or signs
	be a signal for or a symptom of