Do you remember the last shot of Norman Bates where he has a smile on his face because he believes (actually, "she", the mother believes) he has found a way to fool everyone?
Well, that same look (though less creepy) is on Marion Crane's face when, while driving, she realizes that even if she is in a dangerous situation, she has managed to do what they didn't expect. She has taken the money and the poor people who thought they knew her are right where she left them, wondering how that thing could have happened.
We first see her worried but a moment later she lets herself feel proud because, for once, she acted like a clever, nasty girl and is pretty close to getting away with it. Well, let's leave it at that so that we won't rain on the poor girl's parade.
PS: Yes, I can post pictures of people who are not named "Leigh". I just don't want to ;)