When Roland and his ka-tet went to todash they saw dead people there. I am sure you remember Susannah and Oy noticed them first...

I am simply interested in your thoughts and the logic you see behind their existance.
How did they get there? I mean do they walk around us all the time but we cannot notice them or they could be there because Roland and his friends opened some gate or door... were they just lost visitors?... did they have to leave our world when the ka-tet had to go or they could stay? We know they heard the unbearably beautiful sound of the kammen.