Koenig, 41, who played Richard "Boner" Stabone on the 1980s sitcom, disappeared last week and had reportedly been battling depression.
Lance Miccio, who has known Koenig for seven years and collaborated with him on multiple projects, said that early this month, the actor told him he didn't want to work anymore.
"He's been my editor on about 15 projects. I told him, 'Look, I have a new gig, are you interested in working?' And he said 'No, I'm not,'" Miccio told ABCNews.com. "Then I invited him out for drinks to talk and see where his head was at, because I knew he wasn't feeling great, and he declined on that."
Then, while Miccio was out of town, Koenig then dropped off a bag of belongings and gifts at Miccio's door.
"I was away, I was in Florida, and he dropped off a bag of things of mine that he had," Miccio said. "He hooked it on my doorknob -- a plastic bag of video tapes and stuff we had worked together on. Even gifts I had given him. He just returned everything, with no note or anything."
Miccio speculated that Koenig's trip from his Venice, Calif., home to Vancouver, where he once lived, may have had a dark motive. He noted that the 41-year-old actor "suffered with depression and "would get down easily."
"He liked to go to Vancouver because it's beautiful and quiet, it's his favorite place in nature," Miccio said. "One of the films we did together, 'Living the Blues,' it's about a manic depressive musician who went to a spot in nature that he loved and took his own life. I'm not saying that's what happened with Andrew, but it has occurred to me."
According to police in Vancouver, Koenig was visiting friends in West Vancouver when he disappeared. He was scheduled to return home to California Feb. 16; police were notified two days later when he didn't arrive.
"Andrew has recently been despondent, and his family and friends are concerned for his well-being," police said in a statement released Sunday. "He is white, 5'5" tall, 135 lbs., with long, brown shoulder-length hair and brown eyes."
"Andrew, who once lived in Vancouver, liked walking in Stanley Park," the statement continues. "Anyone who sees Andrew or has information about his whereabouts is asked to call 911."
Koenig's parents told TMZ.com that their son was "suffering from depression" and was "not doing good" prior to his disappearance. Judy and Walter Koenig, who played Chekov on the "Star Trek" TV series and movies, said they last heard from their son on Feb. 9. They believe his cell phone is currently turned off and that he was last seen at a bakery in the Stanley Park area of Vancouver.
ABCNews.com's calls to Koenig's parents were not immediately returned Monday.
Hollywood has taken to Twitter in an attempt to find the actor. Stars including Alyssa Milano, Sarah Silverman, Alyson Hannigan, Greg Behrendt have publicized Koenig's disappearance with their tweets.
Friday, Silverman (@SarahKSilverman) tweeted, "ANDREW KOENIG was last seen in VANCOUVER on Feb 14. If you know his whereabouts, tweet #andrewkoenigmissing."