A lost item; an article that has been misplaced or lost (often used in official/lost-and-found contexts).
駅の窓口に紛失物の届け出をした。
I filed a lost-item report at the station ticket office.
紛失物は遺失物取扱所で預かっています。
Lost items are kept at the lost-and-found office.
空港でパスポートを紛失物として届け出た。
I reported my passport as a lost item at the airport.