Refers to a person whose whereabouts are unknown, often used in police or news contexts. 'Runaway' is a possible nuance but not the primary meaning.
The police are continuing the search for the missing person.
Her name was on the missing persons list.
Compound of 失踪 (しっそう, 'disappearance') + 者 (しゃ, 'person'). 失踪 is composed of 失 (lose) and 踪 (footprint, trace), literally 'lose one's traces'.