Until recently, Ly'leth was sending magical messages to us regularly from within the Nighthold. She took care to hide these messages - only the most vigilant scryers could have detected them.
They could not be within the palace... too much interference from the Nightwell...
Of course! The Terrace of Enlightenment! The Duskwatch must have set up a listening post there.
Check the scrying orbs for traces of Ly'leth's messages. We must learn what she was trying to tell us!