Another thing that is not well done by Hitchcock in this film, it's like Jeanne Dielman in this sense also, both films are based in huge cities. Yet, Hitchcock has Jimmy Stewart following Kim Novak all over San Francisco, until this scene, unobserved by Novak or anyone else! She goes into a museum, it's empty; he follows her into the museum, empty except for the two of them. I live in a big city and I worked with detectives in a big city. Of course, they tailed people all the time unbeknownst to to the subject. The easiest place to tail a person unobserved is in a big city with a zillion people around. The hardest place, the place you're most likely to get discovered, of course, is when the subject is alone.