They don't fuck around with aviation investigations.
Whoever did it better leave absolutely no evidence and melt the gun before the alphabet squad comes pounding the door in.
That or it's just a lone wolf good ol boy who deeply regrets his medication made him do that, and his pain and suffering from guilt is punishment enough.
It was actually XT-15 that started drawing on the next clone, he had a bit of a rebellious phase in the vat he never quite grew out of.
He just told 16 that it was tradition and his just faded over time. It's not like the guy fresh out of the jar knows tattoos are permanent... Yet.