紛失 and 盗難: a missing item and theft as its cause
Distinguish losing a ticket from theft as a cause. An item being missing alone does not establish theft; also separate prevention purposes from actual events.
Read the lost-item state
紛失, read ふんしつ, means loss or misplacement. 切符を紛失して駅員に相談した。 says that someone lost a ticket and consulted the station attendant.
Losing the ticket and consulting the attendant are reported. This information does not itself supply another person stealing it as the cause.
Read the purpose of preventing theft
盗難, read とうなん, means theft. 自転車の盗難を防ぐため鍵を二つ掛けた。 says that two locks were put on the bicycle to prevent theft.
Locking is completed, while 防ぐため gives the prevention purpose. It does not report an already stolen bicycle or complete elimination of later theft risk.
Prevention and an event noticed
資料の紛失を防ぐため番号を付けた。 says that the materials were numbered to prevent loss. Distinguish this purpose from an actual loss event.
宿で盗難に気づき、すぐに係員へ知らせた。 says that someone noticed theft at the inn and immediately informed the staff. Noticing and promptly reporting are both stated.
Does an item being missing alone establish 盗難?
Show the answer and why
No. Separate the missing state from theft as its cause. Another person stealing it needs additional evidence.