Utopia Club - Rorschach hides his mask in an alley outside this club. It must appear more often. Check.
Double R/RR/similar - Often appears as "reversed Rs".
Rorschach often signs his name with two lowercase "r"s, back to back. (This figure can be created by drawing a single "r", then folding the page over to duplicate it, similar to the creation of a rorschach-style inkblot.)
Chapter V: Two reversed uppercase "R"s appear on the sign (for the Rum Runner) outside Moloch's apartment. They are made to resemble a skull-and-crossbones. These are themselves reflected in the puddle on the ground.
XII:31:2 - The New York Gazette has the headline "RR to run in 88?", presumably in reference to Robert Redford.
X:28:5 - Bubastis purrs, which appears in her speech bubble as "RR?"
In Chapter I in Watchmen: The Motion Comic, the double R in the middle of the neon sign above Happy Harry's blink.