Because a Forsaken can also use Travelling and make herself invisible with the OP.

Novices have chores and training. It doesn't make sense for Mesaana to pose as a novice, if she can be an AS. Besides, RJ and BS gave broad hints that Mesaana poses as an AS. And actually it was already indicated in Alviarin's PoV in ACoS, prologue:

"The logic was clear, and stunning. Mesaana hid herself because she might be recognized. She must reside in the Tower itself. On the face of it, that seemed impossible, yet nothing else fit. Given that, she must be one of the sisters; surely she was not among servants, bound to labor and sweat. But who? Too many women
had been out of the Tower for years before Elaida's summons, too many had no close friends, or none at all. Mesaana must be one of those. Alviarin very much wanted to know. Even if she could make no use of it, knowledge was power."

Norine perfectly fits that bill.