(law) An individual yet to identified.

plural of person unknown

a phone number (on a phone call) that is unfamiliar to the recipient / a hidden number; a phone number that is hidden from the recipient

(archaic) Alternative form of unbeknownst

(dated, rare) Mysteriously, without a known reason or cause. / (rare) Obscurely, without being known or noticed. / (uncommon) Alternative form of unknowingly, without knowledge; without intent.