No they don't disappear; you can safely leave items wherever you want, and they will still be there when you go back.
EDIT: Do not take over Malcolm's house in Verdistis before you expose him; when he is arrested and the house locked up, any items inside will be removed. Be careful of renting rooms; if you miss a payment, the room will be locked and the items inside unavailable until you re-rent the room.
Last edited by Raze; 25/03/03 07:02 PM.