As a thought experiment, imagine that the city government was motivated to find a car's owner for some reason, perhaps as a suspect or witness of a serious crime etc. Meaning if they have your plate number and think you're a murderer, they will definitely find you and communicate with you (the message that they communicate will be "come to the police station").
I claim this is a proof by construction that they could communicate with you by some method other than posted signs, if they actually wanted to. And they just choose to do the laziest possible "post signs" solution because of basically incompetence/laziness, not because there's no other way for them to identify and communicate with a car's owner.