In my office (at least on my team), we often dress up each other's desks when it's a birthday or other special day.
Meghan went on vacation last week (she sits next to me) and I decided to dress up her desk as a "Crime Scene".
She knows nothing of this and won't until she shows up at work tomorrow (I won't be there, darn it all).
I first put up the "Missing" poster a couple of days after she left and gradually added the rest.
In her chair is a mat with the "chalk outline of a body" on it.
I hope I don't end up in HR... :)