in hiding vs out of sight

in hiding is not an adjective while out of sight is an adjective.

Word	Noun	Adjective	Verb	Adverb
in hiding	No	No	No	Yes
out of sight	No	Yes	No	Yes

As adverbs, in hiding and out of sight are synonyms defined as:

Other synonyms of in hiding include doggo.

in hiding (adjective)	out of sight (adjective)
	not accessible to view

in hiding (adverb)	out of sight (adverb)
quietly in concealment	quietly in concealment
	no longer visible