The agent, or someone who performs the action ("X" in Structure 5 above), is not mentioned here because it is unimportant or unknown or obvious from the context.
会議で予定が 発表されました。
The schedule was announced at the meeting.
←会議で予定を 発表しました。
[They] announced the schedule at the meeting.
東京でオリンピックが 開かれました。
The Olympic games were held in Tokyo.