Which two pieces of evidence give the impression of a secret society?

From the text:

Holmes walked across to the desk and dabbed a little ink from each bottle on to the blotting paper. “It was not printed in this room,” he said; “this is black ink and the other purplish. It was done by a thick pen, and these are fine. No, it was done elsewhere, I should say. Can you make anything of the inscription, Ames?” “No, sir, nothing.” “What do you think, Mr. Mac?” “It gives me the impression of a secret society of some sort; the same with his badge upon the forearm.” “That’s my idea, too,” said White Mason.