Sheila's young son Peter tells Cathy that he had been awakened by a gunshot on the night Mason was murdered. Unaware of the affair between her step-mother and Dr. Rivera she goes to Rivera's office and tells him that Sheila must have killed Mason. Rivera tries with increasing desperation to talk her out of it and, confused, she rushes out.
Then ... Rivera is even more confused; he wanders around town with the Hippocratic oath reverberating inside his head - an oath he has twice violated. We see him here in an open park area with a Western Airlines ticket office behind him.
... and Now, this was filmed in Union Square. The view looks from the square to the 200 block of Geary Street. The square's layout has changed significantly and the Western Airlines office at 287 Geary (map) is now a barber salon, The Art Of Shaving - it's name is inscribed within the yellow oval.
... from the early 1960s ... this vintage postcard image shows the Western Airlines office, at far right, as it was when the movie was filmed. I. Magnin is at the left end of the block with Macy's/Blums and Dohrmanns in the center
Then ... He wanders up a narrow lane with the banner behind him revealing that this is the two block Maiden Lane that runs between Union Square and Kearny Street. He is walking west towards Union Square - that's Grant Avenue crossing behind him (map).
Then ... As he approaches Stockton Street Rivera darts into a doorway when he sees a cop passing by. The entrance to the garage under Union Square is visible across Stockton. Interestingly, this same location was seen ten years earlier in the 1950 movie Woman On The Run.