Night Moves (1975) on Netflix

Harry Moseby is a former pro football player turned Los Angeles-based private investigator, who owns his own independent firm. What he thought was his stable marriage to high-end antiques gallery manager Ellen Moseby is not, when he finds out she has been having an affair. Although she still loves Harry, Ellen believes his work - especially having to do all his own legwork in his independent agency, as opposed to working for a larger private investigation firm - is overtaking his life and their marriage. This revelation is bad timing for Harry as he starts on a new case, to locate two-week missing sixteen-year-old Delly Grastner for her mother, twice-divorced Arlene Iverson, a former bit actress. Beyond the job itself, Harry can see that Arlene's motives are not motherly love. He eventually learns that Arlene, who lives well, achieved what little success she had in the business by sleeping with the right people, and that the probable reason that she wants Delly back is that her daughter's sizable trust fund set up by Arlene's first husband, Delly's late father, is Arlene's sole means of support. When Harry believes he discovers not only Delly's reasons for running away but also to where she was running in achieving her goal, he changes the complexity of the case solely from missing person to possibly smuggling, attempted murder and multiple homicide, which have far more inherent dangers to himself. All the while, he still grapples with what to do about the broken state of his marriage.