Stars are a symbol of greatness and are very important to us (for example the polar star and the christmas star). This might be why they chose a star.
I also find it interesting that this text is so large in the throne room. It might be a phrase uttered by Zaros and then placed in it's throne room, like "know thyself" at Apollo's temples in real life.
Zaros has left Gilinor for a while, so his throne room might have become the most important temple in the empire, instead of the place where their ruler lives. And if that is true the phrase might be something like "Big brother is watching you". Because Zaros is hidden in the stars and sees and knows what you do.
A second meaning could be that it is a metaphor for Zaros' ideology. As he wants people to better themselfs and find balance within a person and thus reach for the stars.
Also note that the -a suffix of "abscondit-a" signifies that whatever it is that is hidden in the stars, it's female. Or at least, the Infernalatin noun is female. A more accurate translation would perhaps be: "She is hidden in the stars."
Either that, or Jagex hasn't been paying attention to their Latin grammar.
In that case it could be Mah, as only the Mahjarrat, Zaros and Seren knew where she was, or that she was real, this could be considered as 'Hidden in the Universe/Stars', as very few even knew she existed or where.