So I know that it completely looked like Sirius murdered Peter and he was a Death Eater. But this has always bothered me... Why didn't they check his arm for a Dark Mark? Are they only visible when Voldemort is strong, because I remember in GoF Snape talking about the mark getting darker. If that is the case, I get why they didn't check. But if not, wouldn't that have been the easiest way to figure out if he was truly a Death Eater? On another note, if Peter did get a Dark Mark while being a spy, he did a good job of hiding it from his friends.