Apparently it's new, a lot of people think it's a myth. But people have said they've seen people named Thief leaving their house or the area of it and noticing things missing. As such, I'd seriously prefer knowing what's safe to put things in. I thought only Safe boxes worked, but UESP said something about Wardrobes being safe, too. So I want to be sure I can dump all my collectors crap in there before continuing to Riverwood to retrieve the rest of it.