store detective vs shamus

store detective and shamus both are nouns.

Word	Noun	Adjective	Verb	Adverb
store detective	Yes	No	No	No
shamus	Yes	No	No	No

As nouns, shamus is a hypernym of store detective; that is, shamus is a word with a broader meaning than store detective:

store detective: a private detective employed by a merchant to stop pilferage
shamus: someone who can be employed as a detective to collect information and assist in criminal investigations

Other hypernyms of store detective include operative, PI, private detective, private eye, private investigator, sherlock.

store detective (noun)	shamus (noun)
a private detective employed by a merchant to stop pilferage	someone who can be employed as a detective to collect information and assist in criminal investigations