When Cullen arrives at his lover Lois's home she tells him her husband is planning to kill her for her money. While he was at the airport on a business trip she had found his hidden gun but as they speak the husband sneaks through the back door intent on carrying out his plan. She confronts him, the gun in her hand goes off and he falls dead.
Then ... Cullen is a homicide lieutenant. Does he arrest her? No way, not in the noir world, instead he decides on a cover-up. They load the body in his car and he drives off to dump it at the airport, then known as San Francisco Municipal Airport (map), a logical place given that he was supposed to be there.
... and Now, the terminal above, built in the 1940s, was replaced in 1954 by a new structure, the Central Terminal. It's still there, below, now called Terminal 2. The airport was renamed San Francisco International Airport in 1955. (The same terminal was seen a year earlier in the movie Impact).
Cullen deposits the corpse at the airport and takes off, unfortunately witnessed by a couple in a passing car.
Then ... He decides a perfect place to dispose of the murder weapon would be the deep waters of the San Francisco Bay under the Golden Gate Bridge. But when he pulls up to the toll booth he is recognized by a fellow cop. His plan is off to a rocky start ...
... and Now, the same booths are still very similar, they have been renumbered and there are some cosmetic changes but the biggest difference today is the absence of toll-collectors, displaced in 2013 by Fastrak sensors and license plate cameras.
Then ... Later in the movie we get another, daytime, view of the toll booths.
... and Now, there's still a clock mounted in the center.
Cullen pulls over on the bridge (today's traffic wouldn't allow that) and throws the gun over the railing, into the bay. Now he feels better, with no weapon any investigation will be severely hobbled.