noun
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.
警察は失踪者の捜索を続けている。
The police are continuing the search for the missing person.
彼女は失踪者リストに名前が載っていた。
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'.