1. The window was looked at by Mary.

Does #1 sound not OK?

Is there any sentence that sounds OK including is/are/was/were looked at?
Sorry, 1. does not sound OK.

Mary looked at the window.

Mary looked through the window.

Mary looked out of the window.

Many verbs do not or only rarely appear in passive voice even though it is possible to construct such, moon. The verbs of sensation are some of them.
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The passive voice is more common in formal English, especially if the meaning is to 'consider'. Here is an example: The proposal was looked at by the Council.
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