Anonimo said: "You have a tattoo," Sherlock said surprised. "Yeah, I figured you deduced it ages ago," John replied. "Nope, is it just the one?" Sherlock asked. "No, I've got quite a few," John told him grinning wickedly, "care to guess where they are and what they look like?" Sherlock spent the next hour deducing and uncovering all of John's tattoos save one. "One left, you can only see it under a black light," John told him. It was over his heart and it read "Could be dangerous. SH"