忘れ物 (wasuremono) means “lost item, thing left behind”

More examples

電車に忘れ物をしました。den sha ni wasu re mono o shi ma shi ta

I left something on the train.

ホテルを出る前に忘れ物がないか確認しました。ho te ru o de ru mae ni wasu re mono ga na i ka kaku nin shi ma shi ta

I checked for anything I had left behind before leaving the hotel.

Describes an item that someone unintentionally leaves behind or can no longer find because it was forgotten.

忘れ物をするwasu re mono o su ru
leave something behind
忘れ物を探すwasu re mono o saga su
look for a lost item
忘れ物がないか確認するwasu re mono ga na i ka kaku nin su ru
check for anything left behind

落とし物otoshimono: An item that is dropped and left behind unintentionally.
紛失物funshitsubutsu: An item that the owner has lost and cannot find.

うっかり置き忘れたり、なくしたりした物。u kka ri o ki wasu re ta ri na ku shi ta ri shi ta mono

The noun 忘れ物 (wasuremono, "forgotten thing") combines 忘れ (wasure, "forgetting") from 忘れる (wasureru, "to forget") with 物 (mono, "thing").

In everyday conversation, this word is used when someone reports finding or looking for an item left at a station, school, or other place.