Is Sajdah obligatory even if one does not know it is a verse of Sajdah?
It is commendable to recite the verse of Sajdah silently, therefore the reciter should keep this in mind. But if someone recites it loudly and others hear it, then Sajdah al-Tilawat becomes obligatory, whether the listener knows that this verse is a verse of Sajdah or not. Because there are three causes for Sajdah al-Tilawat becoming obligatory: (1) Recitation (Tilawat), (2) Hearing (Samaa'), (3) Following the Imam (Iqtida). If any one of these causes is realized, Sajdah al-Tilawat will become obligatory. Furthermore, there is also a generalization in hearing that whether there is an intention to listen or not, Sajdah al-Tilawat becomes obligatory in any case.
(Book: Habib al-Fatawa / Vol: 2 / Pg: 241)