Keep out!

We had a play date here this morning with a few friends and so I told the kids they needed to pick up their rooms. Max is actually pretty decent at cleaning up. (if throwing everything from the middle of the floor behind his chair constitutes cleaning up) Today he said he put a sign on his door while he was cleaning to let me know I was not allowed to come in. See below. I have no idea where he learned this, but I didn't think it started until middle school when they put keep out signs on their doors! What am I in for...??!?