How does Nick meet Tom's mistress in The Great Gatsby?

In chapter 2 Tom asks Nick to take a ride with him into the city.  On their way through, they stop at George's garage where Tom and George discuss a car that George could really use.  When George leaves the room for a brief moment to get a chair for them to sit on, Tom makes arrangements for Myrtle to sneak off to New York on the next train.  He does this right in front of Nick.  So Nick is introduced right away to the fact that they are having an affair.  Earlier it was one thing to know that he was taking phone calls at his own home from "another woman," but this was definitely a very blatant move that helps create the true character of Tom according to the way Nick sees him.