I got a red paper with the following message when I took my bicycle in front of a station in Japan.
無断{むだん}利用{りよう}されています。
It roughly means "being used without authorization".
What is the subject of the sentence? Does it refer to the space on which I parked my bicycle?
Why was it phrased using passive form?
Is there a convention in Japanese language to select the implicit subjects?
The full sentence would be:
あなたはこの駐輪場を無断利用されています。
The omitted subject is "you" and the omitted object is 駐輪場【ちゅうりんじょう】 (bicycle parking), where you parked your bicycle. Many bicycle parking areas near stations are operated by local governments and available only for registered users who paid annual fee.
This れる is not a passive marker but a honorific marker. The message seems politer than it needs to be. 「(あなたは)無断利用しています」 would have been equally OK in this situation.
It's theoretically possible to take this sentence as a passive sentence (i.e., この駐輪場はあなたに無断利用されています), but as you already guessed, passive voice is not necessary here, and inferring an implied subject other than 'you' is not necessary, either.
