Someone drank from the Well. If the DR was performed, that's how she found out about Kieran. So she's connected to whoever drank from the Well. She'd know the orb was destroyed regardless of who drank. Also, if it IS her orb then I would assume she'd know it was destroyed. Also it's possible the game files simply refer to it as Mythal's orb because at some point in the development cycle it might have been meant to be hers and then the devs changed their minds.
At any rate, with the orb out of the way she can probably assume that Fen'Harel will seek another source of power. She probably knew when he awoke even if he wasn't aware that she was still "alive". For all we know, she's the reason the orb gave the mark to the Inquisitor. Really Flemeth seems to know most of what's going on at any given time or she manipulates events to happen the way she wants.