What if there are robbers?

On Friday I asked my students if they thought an author was going to sneak into our classroom this weekend and paint another character onto the desk. They threw out quite a few guesses as to who might appear, but then a student asked how the artist will get in if the door is locked.

Me: Well, I can leave the front door unlocked so that an author can sneak in and paint.
Student: But what about robbers? We don't want any robbers to steal our stuff.
Me: I know what to do. I will write a note that says if you're a robber, don't come in. If you want to paint something onto our desk, please come in.
Student: That's a great idea. 

I'm glad we made a plan. Who's it gonna be?