Obviously we can not know for certain what they were thinking, but with some knowledge about ancient interpretations of this text and the translation philosophy employed by the LXX, I suspect someone around here has a better idea than I do.
כֵן יִתֵּ֨ן אֲדֹנָ֥י ה֛וּא לָכֶ֖ם אֹ֑ות הִנֵּ֣ה הָעַלְמָ֗ה הָרָה֙ וְיֹלֶ֣דֶת בֵּ֔ן וְקָרָ֥את שְׁמֹ֖ו עִמָּ֥נוּ אֵֽל׃
διὰ τοῦτο δώσει κύριος αὐτὸς ὑμῖν σημεῖον· ἰδοὺ ἡ παρθένος ἐν γαστρὶ ἕξει καὶ τέξεται υἱόν, καὶ καλέσεις τὸ ὄνομα αὐτοῦ Εμμανουηλ·
My understanding is that עלמה has a semantic range that includes "virgin" but is less specific, hence the RSV "young woman." The Greek chosen by the translators of the LXX - παρθένος - however, is more specific to lack of sexual experience.1 I don't have a better Greek word (suggestions welcome), but I would expect the context would compel the translators to find a different way to express this.
I can think of a few possibilities. I am wondering which is most likely or if there are others I haven’t thought about.
They LXX understood their own translation as
Behold, the virgin [having forsaken her virginity] shall conceive....
where the bracketed clause is obvious and therefore assumed.
They were using a different Hebrew text that contained a Hebrew word whose range lines up more closely with παρθένος.
Τhey were intentionally adding a miraculous element of virgin conception to this prophecy.
The semantic ranges of these two words were other than what I summarize in the footnote below.
They misunderstood עלמה.
Please note: This question does not ask how many times one believes this prophecy has or has not been fulfilled. While it may be relevant to speculate about what the LXX translators thought about the question, that is as far as I’m interested for the purposes of this question.
1. BDAG παρθένος (italics original): “one who has never engaged in sexual intercourse, virgin, chaste person."
HALOT עלמה: “a girl who is able to be married…young woman”
2. I think the Hebrew may be ambiguous as to whether she is already pregnant. (I have asked another question about this). However, because the Greek is clearly future tense, I don’t think this precludes this understanding of the Greek.