IOU definitely does mean something. After all, this show is all about cases so every detail means something. In season 2 episode 3 there's this scene where sherlock is analyzing something in a microscope at the lab, and he mumbles, I o u.. After Molly questions him about it and he just replys with, "nothing, just a mental note". I could write a whole essay about I ou haha so I'll just leave it there. Just search on google what does I o u mean in sherlock and you'll find a good theory