وُجِدَتْ رِسَالَةٌ فِي صُنْدُوقٍ.
A letter was found in a chest.
Wujidat is a passive perfect verb; risālatun functions as the grammatical subject of the passive. The discovery and the discovered item are foregrounded, while the discoverer's identity is not expressed. This does not establish that the discoverer is unknown.
By contrast, wajada al-bāḥithu risālatan names the discoverer. If the audience disputes who found the letter, that active construction may suit the occasion better. Situational fitness determines the preferred wording without necessarily changing the underlying event.