According to eyewitnesses, McKenzie visited Baccarin multiple times on the Vancouver set of her film Deadpool -- which shot between March and May of this year.
"He stayed at her condo," a source tells ET.
Baccarin films Deadpool night scenes on April 1 in Vancouver, Canada.
In court docs, Baccarin claimed that "In March 2015, Austin and I were talking about separating as I told him I thought our marriage was not working and we should possibly divorce."
Chick did file for divorce from his wife of four-and-a-half years on June 12, citing "irreconcilable differences." According to a source, a fan sent Chick a photo of Baccarin and McKenzie together in Vancouver.
"Austin confronted Morena about the photo," the source says. "She didn't deny it and said she no longer wanted to work on [their] marriage."
Prior to the divorce, in the spring of 2015, Baccarin and Chick had reached an informal agreement regarding custody and visitation of their one-year-old son, Julius, wherein Chick would be provided with a New York apartment and spend considerable time with the child to accommodate Baccarin's eight-month shooting schedule on Gotham.
On Thursday, the former couple faced off in court once again -- on the same day that news of Baccarin's pregnancy went public -- regarding custody of their son.
Get the details on the tense face-off in the video below.
ET has reached out to reps for all three parties for comment.