My son was meant to clean his room. He did not. And when my wife found out, she was slightly annoyed. To make things worse plastic bottles were found down the side of his bed. When challenged about this, he said it was not him.
Talking about it at work, the same issue happens with a fourteen year old son of one of the ladies who I work with. All sorts of things appear in his bedroom which he did not put there either. So we worked out that there were two options. One is that my daughter has put things in my sons doom and has gone to the house of a boy she does not know to put things in his room. Or that it was aliens. No other explanation is really possible.