There's a great scene in Harry Potter and the Natural 20 where they find the Mirror of Erised, and Milo the Human Wizard gets to use his ranks in Code Breaker, which he's never used before. He busts out the pen and paper, ready for a difficult d20 check that will test the limits of his stats. And then he realises it's just backwards. He's so disappointed.
Lots of them, all scattered across the body. And those tattoos should be investigated by the FBI agent whose name is one of the enigmatic tattoos. They're also expected to be deciphered at the right moment by a brilliant FBI mathematician. (Spoiler: there are also tattoos using invisible ink that can only be seen when the person visits a place full of UV lights). It takes time.