unknown vs unbeknown

unknown is a noun but unbeknown is not a noun.

Word	Noun	Adjective	Verb	Adverb
unknown	Yes	Yes	No	No
unbeknown	No	Yes	No	Yes

unknown (noun)	unbeknown (noun)
a variable whose values are solutions of an equation
an unknown and unexplored region
anyone who does not belong in the environment in which they are found

unknown (adjective)	unbeknown (adjective)
not known	(usually used with `to') occurring or existing without the knowledge of
being or having an unknown or unnamed source
not famous or acclaimed
not known before
not known to exist

unknown (adverb)	unbeknown (adverb)
	without someone's knowledge