「ひょっとして、今まで気づいてなかったのですか」「いや、まぁ・・・。想像の埒外だったもんで・・・」

「、」「、・・・。・・・」

'By any chance, did you not realize until now?' 'Well, no, it was... kind of beyond what I imagined.'

Literal

''Maybe, until-now not-realize-[state-ていた] [question-のですか]?'' ''No, well... imagination [possessive-の] outside-bounds was-[because-もんで]......''

埒外 (らちがい) literally 'outside the enclosure' means 'beyond the scope, out of range, inconceivable'. 埒 (らち) is an archaic word for 'enclosure, fence, boundary' — mostly preserved now in this one compound and in 埒があかない ('getting nowhere'). もので (colloquial もんで) is the causal conjunction 'because, given that', slightly more formal/explanatory than から. The ellipsis (・・・) at both ends marks hesitation.