The dark mark & Sirius

[vheggie at yahoo.com]

No: HPFGUIDX 24784

This dialogue occurs towards the end of Chapter Twenty Seven (Padfoot 
Returns) in GoF

>"Yeah, you should've seen Snape's face when Karkaroff turned up in 
Potions yesterday!"  said Harry quickly.  "Karkaroff wanted to talk 
to Snape, he says Snape's been avoiding him.  Karkaroff looked really 
worried.  He showed Snape something on his arm, but I couldn't see 
what it was."
He showed Snape something on his arm?"  said Sirius, looking frankly 
bewildered.  He ran his fingers distractedly through his filthy hair, 
then shrugged again.  "Well, I've no idea what that's about. . . but 
if Karkaroff s genuinely worried, and he's going to Snape for 
answers ..."<

This puzzled me - does Sirius not know that the Dark Mark appears on 
the arms of the Death Eaters?  Is this not common knowledge (you'd've 
thought that all the trials would have brought this to light)?
Or perhaps he's not aware that Snape was a Death Eater (again - 
you'd've thought that the trials would be common knowledge - or was 
Snape tried after Sirius went to Azkaban?)

Any other thoughts?