The soon-to-be-exes both showed up Monday night at a screening of Hector And The Search For Happiness in East Hampton.
Paltrow, accompanied by her mother, Blythe Danner, hosted the evening for her and Martin's longtime friend, Simon Pegg, who stars in the film.
"They seemed so comfortable with each other," says an observer. "It was like they were still married."
Paltrow arrived for the screening at the UA Theater in East Hampton, posing for pictures and entering the theater by herself, while Martin entered separately through another door.
They chatted with friends before and after the screening before heading to the after party at Serena and Lily Beach Market in Wainscott.
Martin spent time talking to friends with Paltrow and sitting at a table with his mother-in-law. He chatted with friends including Pegg, who is godfather to Apple, his daughter with Paltrow.
"We go back a long way," Pegg told PEOPLE. "I've known Chris for 15 years and I was there when they met and they are dear friends. And Chris is the godfather to my daughter, Tilly, as well. They are both so down-to-earth."
As the night wound down, Martin motioned to Paltrow that he was ready to leave. They said goodbye to their friends and left – with Martin walking Danner out of the tony store where the fete was held. He escorted her all the way to her car, adds a source.
"I don't know what's going on with them," said another observer of Paltrow and Martin, "but they seem really happy."