I absolutely don't think it should be formally recognized as being limited to the previous sentence, though.
It could be anything that could be referred to... but it probably isn't. That's not how people determine things from context. They start with the things that are directly relevant to the conversation in its current moment: the topics being discussed, the events and things surrounding them, other people present, things that have just been said, things that the participants are physically indicating or interacting with. Those are the sorts of things that are generally assumed to be always salient. That is, if a referent or antecedent isn't explicitly present, it can probably be assumed to be one of those things. (There's probably a more formal or precise way of phrasing that, but I'm not a linguist.)
If I'm at a sporting event, and the opposing team scores, and the friend I'm with grumbles, "Ah, this sucks!", what is the referent of "this"? How can I figure out what sucks? It could be anything: the stadium the game is taking place in, the view from the nosebleed seats, the ache in their knee that's been acting up ever since we had to walk up to our lousy seats, the half-eaten hot dog sitting in their lap on a plate, the ambient weather conditions, an unpleasant childhood memory that crossed their mind, or just general dissatisfaction with the world in general. However, because I have had conversations before, I can conclude that they are probably referring to something that is immediately relevant to the moment: the fact that the other team just scored.
Like Klingon, English has linguistic constructions for explicitly connecting pronouns to their antecedents. My friend could have said "Ah, the other team scored again, and that sucks!" But because my friend is aware of how people usually interpret utterances in context, they didn't need to. "Ah, this sucks!" sufficed. On the other hand, if my friend had wanted to complain about something not immediately relevant, they would know that they probably need to introduce it explicitly before complaining about it. "Ah, I just remembered traffic on the ride home is going to be awful! This sucks!" or the like. From a Klingon perspective, that's where constructions like 'e' or ngoDvam come in handy.
This is what I'm getting at. Obviously, an unstated third-person subject or object is not guaranteed to refer to the previous statement, and I don't think there should be a rule to that effect. My argument is that, when Klingon speakers are trying to figure out the intended referent from context, "the previous sentence" is going to be considered as a possibility long before something discussed three hours ago, or something entirely unrelated. The point of trying to find canon citations was to support the idea that
Klingons also have "the previous statement" in their mental category of
immediate referents. Of course, there are other referents that would probably be considered first, like explicit third-person nouns, or something the speaker is pointing at. But I think "the previous statement" is much closer to the "explicit third-person nouns" end of the relevance spectrum than it is to the "Voldemort's name" end of the spectrum.
If mayqel starts a conversation by saying to me, bIvoqbe', 'ej muSuj, how will I deduce what the subject of muSuj is? It's not referring to a third-person thing or person mentioned earlier in the sentence, because there are none, and there are no earlier sentences in our conversation to look back to. It's not referring to a third-person thing he's indicating physically, because this is the Internet and we both know I can't see if he's pointing at something. What's something else immediately relevant he could be referring to? Well, the thing he just said. It makes sense that the fact that I don't trust could be something that disturbs him, and the fact that the sentences are connected with 'ej suggests at least some rhetorical connection. So I'll go with that. Yes, it's possible that he's disturbed by (say) the signs that Voldemort has returned, and that's what the subject of muSuj refers to, and for some reason he expressed his concerns in conjunction with an unrelated thought. But I know that participants in conversations try to say things that are obviously relevant (unless Maltz has some information on Klingon conversational maxims he hasn't been asked about), so it's probably not Voldemort.