Mr. Monk and the Three Julies - S6-E13
Other mistake: When Natalie sits down with Randy eating ice cream, she's says "vanilla fudge, my favorite." But the carton is labeled "chocolate." Plus Randy says he got it out of her freezer, so she would know she bought just regular chocolate ice cream.
Answer: Notice also that in the opening credits scene when he is touching parking meters her also wipes his hand on his clothes each time he does it.