There is a class of verbs that have both transitive and intransitive forms, in which the object of the transitive form is the subject of the intransitive. These are sometimes called labile or ergative verbs. The usual example is "to open":
John opens the door.
The door opens.
The intransitive form implies there is no agent. The transitive form has an explicit agent.
The passive is formed from the transitive verb, and has an implied agent.
The door was opened.
This implies the existence of an unnamed agent.
But we have to be careful with this, because often there is an adjective that looks exactly like a past participle.
So we have "to break":
John broke the window.
The window broke. (the labile intransitive)
A broken window. (an adjective)
The window was broken (ambiguous, could be a passive, but could be an adjective)
In the last case the existence of the adjective means that we can't conclude that "somebody broke the window". The word "broken" could be understood as an adjective and that doesn't imply the existence of an agent.
In the case of "open" the adjective is "open", we say "An open door" or "The door was open" when we use the adjective, so the ambiguity doesn't exist.
Now, while "The window was broken" is syntactically ambiguous, it is not usually semantically ambiguous. The meaning is clear.
How about "connect"? Well it is like "break", because there is a valid as an adjective: "Two connected magnets". So the sentence
The two magnets are connected.
Can be understood as being [subject] [linking verb] [adjective complement] and no agent is implied.