In a passive sentence the agent (the person or thing doing the action) is often left out.
Someone broke the window.
The window was broken.
We could write 'The window was broken by someone' but it sounds clumsy and doesn't add anything to the sentence, so it is better to leave it out.
Sometimes we know who the agent is but it is not important.
Competition winners will be notified in September.
In this sentence it doesn't really matter who is doing the notifying, so it can be left out.
The agent can also be left out of a passive sentence to build up suspense.
The door was pushed open slowly.
In this sentence the author doesn't want the reader to know who is pushing open the door.