紛失 and 忘れ物: loss and belongings left behind
Distinguish the state of losing an item from the item left behind, identifying the finder and owner separately.
The loss of an item
紛失, read ふんしつ, means losing an item. 社員証を紛失したので、担当者に連絡しました。 says that the person responsible was contacted because an employee ID had been lost.
どこで紛失したか分かりません。 says that the location of the loss is unknown. 紛失 alone does not establish whether the location is known; 分かりません states that here.
An item left behind or forgotten
忘れ物, read わすれもの, means an item left behind or something someone forgot to bring. 駅で忘れ物を探しています。 says that someone is looking at the station for an item left behind.
忘れ物がないか確認してください。 asks someone to check that nothing is being left behind. It does not report an established loss.
The finder and the person who lost the item
忘れ物を見つけたので、駅員に渡しました。 says that an item left behind was found and handed to a station employee. The finder is not necessarily its owner.
駅に置き忘れた財布が見つかりました。 says that the wallet left at the station was found. 忘れ物 does not necessarily mean an item that can never be recovered.
Does 忘れ物を見つけて駅員に渡しました。 establish that the speaker owns the item?
Show the answer and why
No. It only states that the item was found and handed to a station employee. Ownership needs separate information.