One issue that prevents translation is just how much passivity is in that sentence. In other languages, more active grammar is preferred, to the point where trying to construct such passivity on purpose would be nigh unintelligible.
For example, in Hebrew, I would translate the sentence as חייב להיות שמישהו היה צופה בה, which literally translates as "it must be that someone was watching her". Trying a more literal take, היא הייתה חייבת להיות צפויה, not only (by sheer necessity) injects the infinitive tense into the middle of the sentence, but also would just cause a native speaker to ask you in English, "why don't you tell me in English what you're really trying to say?"
Whilst I do not know Hebrew your point is very well taken. It aptly demonstrates the sheer complexity of trying to achieve an exact translation between one language and another (in fact from my understanding of the problem an exact translation between most languages is nigh on impossible).
I both admire and pity translators who work for organizations such as the U.N. as they have to translate documents such as treaties and do so with great precision.
However, I suppose my major concern with translations is how sloppy some actually are—that is that errors in translation are not limited by structural limitations caused by differences in the languages as in your example but rather by sheer carelessness. Frankly, I'm fed up with seeing bad translation of subtitles from German into English. I'd be more than happy if I had a dollar for every time I've seen the verbs glauben (to believe) and wissen (to know) interchanged with one another during translation.
Clearly, to know something is very different to believe something but unfortunately it seems that significant numbers of translators find such precision unnecessary.
For example, in Hebrew, I would translate the sentence as חייב להיות שמישהו היה צופה בה, which literally translates as "it must be that someone was watching her". Trying a more literal take, היא הייתה חייבת להיות צפויה, not only (by sheer necessity) injects the infinitive tense into the middle of the sentence, but also would just cause a native speaker to ask you in English, "why don't you tell me in English what you're really trying to say?"