Presumably, Ellis communicates where he might be found before tucking away his crystal.

And he is patiently sat on a low stone wall, watching the dealings of a small cluster of Marchers muttering together. Ellis has chosen the far side of Lowtown, where the city begins to slope downwards, where the lifts are located and operated. The observation of comings and goings has occupied him well enough while he waits for Astarion to arrive.