I assume it means the agent of the verb is what is unknown?
As I'm sure you know, I don't think many Arabs regard MSA as a valid form of Arabic. Whenever I asked any Arab about grammar, they always spoke of CA and never regarded their own dialect or even what we call MSA as "real" Arabic.
I don’t think it is that clear-cut. Many Arabs, usually not well-versed in Arabic linguistics, will by default extol the glories of CA and lament the shame of modern Arabic and its dialects, in this typical Arab tendency to claim that ’we were so much better before’ which they display in pretty much all aspects of their lives. I think it’s just the current Arab mindset which is plagued by the inescapable contradiction that Islam is supposed to be the most advanced way of living and organizing economies and societies, yet the glory of the islamic world is long gone and they are reminded of this on a daily basis.
Yet good MSA is actually very well regarded by highly educated Arabs and I do not recall ever reading or hearing a learned opinion to the effect that MSA is not ’proper’ Arabic. Naguib Mahfouz was heavily criticized for inserting Egyptian Arabic dialogues in some of his novels, but his MSA in other circumstances was never called into question. Even Al-Azhar, which is supposed to be the beacon of Islamic knowledge, communicates in MSA (and out of curiosity I went to their website looking for cases of passive voice with a specific agent, and found a good many instances including, ironically maybe, one about the teaching of Arabic:
مجلة کلية اللغة العربية بالمنصورة مجلة علمية محکمة صدر أول عدد لها عام 1980، وهي تُمول تمويل ذاتي وتنشر أبحاثا متخصصة في العلوم العربية وآدابها والتاريخ والحضارة بعد أن تقوم بتحکيم هذه الأبحاث من قِبل عدد من الأساتذة المتخصصين في نفس المجال.
Al-Jazeera are almost unanimously applauded in the Arab world for the quality of their فصحى, they even dedicate some of their programming to various documentaries and shows on Arabic poetry, ancient Arabic literature and the like. Yet they use من قبل and من طرف constantly. I think that apart from those people who have received a very conservative islamic education where they were taught that anything different from Qur’anic Arabic is wrong, without having actually learned much by way of Arabic literature and linguistics, people who are actually competent in the field readily recognize that Qur’anic Arabic itself is closer to pre-classical Arabic (with usage cases that would be deemed ungrammatical by CA standards) than to CA proper, and that similarly MSA has brought about a better systematization of some rules which for centuries had been left to the appreciation (or individual preference) of writers. The situation I believe is comparable to that of French, where you have some close-minded (and often not that knowledgeable) purists who don’t even really know why a given rule should be kept and a given usage should be condemned, and then you have the likes of Grevisse who actually know what they’re talking about who show them that ’the Emperor has no clothes’. Same with Arabic as far as I can tell.
EDIT: in wanting to give a spectacular example, I read it too fast and now realize it’s not an actual passive form, so here are the other examples I found! :