this can sometimes be a sign of a mental disorder such as an attachment disorder - I would bring him to a counselor - a child doing this for shock value is screaming for attention...
Now that you know who is doing it, you may wish to simply ask him to clean it up. It is important that he know that you are not mad at him, or even act disgusted. He does need to be responsible for this behavior though.
I cant bring myself to think that it's either of those options. I worked in a childcare facility until recently. I witnessed boys this age group intentionally urinating on the floor, walls, and in the sink of the restroom, behind chairs in the gathering room, even found it on the bus after a field trip. I would get so upset because it's disgusting and they(4 brothers that everyone knew were the culprits) would just sit there and act as if they didnt have a clue or care what you were talking about. I wont even get started on the younger age group of boys that do this mess as well.
I think things like this are done for shock value.