My wallet has been stolen by someone.
→ by someone (This phrase is unnecessary because the focus is on the action of the wallet being stolen, not the person who stole it. The sentence would still make sense without this phrase: "My wallet has been stolen.")

We were taught by a different teacher yesterday.
→ by a different teacher (This phrase can be removed because it's implied that someone taught them, and the action is what matters. The sentence would still be clear without it: "We were taught yesterday.")

Nick was operated on at the hospital by a doctor.
→ by a doctor (In this case, the focus is on the operation itself, not necessarily on who performed it. The sentence would still be understandable: "Nick was operated on at the hospital.")

The meal was served by a waiter in a red coat.
→ by a waiter (The phrase "by a waiter" can be omitted because the focus is on the meal being served, and the role of the waiter isn't crucial to the meaning of the sentence: "The meal was served in a red coat.")

We were shown round the museum by a guide.
→ by a guide (The phrase "by a guide" is not absolutely necessary because it’s already implied that someone showed them around the museum. The sentence can still stand without it: "We were shown round the museum.")

Two letters were delivered this morning by the postman.
→ by the postman (The phrase "by the postman" can be removed because the focus is on the letters being delivered, and the sentence works without it: "Two letters were delivered this morning.")

Three men have been arrested by the police.
→ by the police (The phrase "by the police" is unnecessary because it's understood that the police arrested the men. The sentence would still make sense without it: "Three men have been arrested.")

Yesterday a window was broken by someone.
→ by someone (This phrase is not needed, as the sentence is clear without specifying who broke the window: "Yesterday a window was broken.")