To rewrite the given passages in the passive voice, we need to focus on making the object of the active sentence the subject of the passive sentence and vice versa. Let's transform the sentences:
1. A bus has been stolen from outside the school. The thief was seen by some children. The bus is being searched for by the police now. The children's descriptions will be used by them to catch the thief.
2. A UFO was seen in the sky above London last night by some people. It was reported to the police by them. A helicopter was sent by the army to look at it more closely. The helicopter was shot down by the UFO, and both men in it were killed. Photographs of the UFO have been given to the police by people. They are being looked at by experts now.
By using the passive voice, the focus shifts from the doer of the action (subject) to the receiver of the action (object), making the sentences clearer and more formal.