burglar alarm vs warning device

burglar alarm and warning device both are nouns.

Word	Noun	Adjective	Verb	Adverb
burglar alarm	Yes	No	No	No
warning device	Yes	No	No	No

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

burglar alarm: a warning device that is tripped off by the occurrence of a burglary
warning device: a device that signals the occurrence of some undesirable event

Other hypernyms of burglar alarm include alarm, alarm system.

burglar alarm (noun)	warning device (noun)
a warning device that is tripped off by the occurrence of a burglary	a device that signals the occurrence of some undesirable event
a loud warning signal produced by a burglar alarm