A writer can give the reader a grammatical sentence that has no agent, or "doer", of the action of the verb:

A sentence with no explicitly stated agent means the reader has to correctly infer the agent.

In many instances, the inference is not difficult. However, a writer will rarely recognize the cases when the inference is not clear because the writer wrote the sentence and therefore knows who the agent, or "doer", of the sentence is.

Also, see the section on Dummy IT for an interaction between passive verbs and the otherwise unnecessary, sentence-encumbering "Dummy IT".