When I find an item left out I place in the box. When the child discovers they can't find the item they inevitably ask me. I tell them its in the Ransom Box. HAHAHAH! My evil laugh. Now, in order to retrieve said item they have to take a task out of the "Take One" envelope.
Once the task is complete they may have their item. They also put their task in the "Return" Envelope so that its taken out of circulation till all the tasks are used up and then the pile is moved over to the other envelope.
You definetly need one of these. Go make one. Right. NOW. You can thank me later.