The classical expression mā lam yusamma fāʿiluhu literally means the agent is not named. The speaker may know the agent but choose not to mention it.
In فُتِحَ الْبَابُ, the focus can simply be on the opening of the door. The grammatical passive does not prove that the real-world actor is unknown.
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Mafʿūl Mā Lam Yusamma Fāʿiluhu — The Passive Subject
The classical expression mā lam yusamma fāʿiluhu literally means the agent is not named. The speaker may know the agent but choose not to mention it.
In فُتِحَ الْبَابُ, the focus can simply be on the opening of the door. The grammatical passive does not prove that the real-world actor is unknown.