It's a pretty well known fact that cats subscribe to the theory of "if it fits, I sits." Any box, tub, bowl, or any vessel that will fit their body will become their new bed. They especially like hiding in boxes that are mostly closed, so they can jump out at their owners at the last minute.
One owner learned this the hard way after his cat got herself in a tricky situation. She was packing up some boxes in her home to ship to northern Italy, but what she didn't know was that her cat had jumped inside.