Given the following context:

The film focuses on the cast and crew of a new TV series called Manhattan, which is shot on location in New York City. Mel Wexler is a successful producer who has become a workaholic since the death of his family in a plane crash. Trying to forget his loss, he throws himself on producing Manhattan. For the lead role, he tries to cast the feared film star Sabina Quarles, who has a reputation of being hard to work with. She initially declines, explaining she is too good for television. However, because of her past with Mel, she finally accepts the role. They soon start a relationship, but he remains suspicious of her constant visits to San Francisco. She is reluctant to explain why she is going there every month, which makes him think she has an affair.
Meanwhile, playing her stepson in the series is Bill Warwick, an actor who is a sex symbol. He is married to Sandy, but because of his image, he has to keep their marriage a secret. Sandy was once a great actress, but her drug addiction has turned her into a street prostitute, ignoring her work. He thinks a co-starring role could help her going towards the right path, but she fails to show up at her audition. The role eventually goes to Gaby Smith, a rich actress who graduated from Yale. From the very beginning, Bill is upset that his wife wasn't given the role and as a result, treats Gaby very badly. This makes her very sad, constantly trying to win his trust over.

answer the following question:

What makes Mel suspicious of Sabina's constant visits to San Francisco? 

She is reluctant to explain why she is going