I don't know why it does this, but I used to notice it when I'd have a full inventory and cut burlap into strips...they would disappear. Well, after a while of things I set down disappearing, I found a pair of pants I wanted in the shed this time and put them on. I was able to grab 1 item from my black jeans before they disappeared. I went behind the shed, went everywhere I had been, but they were gone. I don't know why, and I've only tested in the red sheds with grass under it, but things disappear completely when laid down.