Should I be concerned?

My husband of 3 years and 9 months, has a woman friend who he works with. He said it's platonic, but he kept the friendship from me for the 3 years we've been married. He would very occasionally call her on a Saturday or an evening from our house "just to talk". I found out about it when I came to his work and they both acted "uncomfortable" when I met her. It gave me some sort of "funny" feeling, so I investigated and found her cell number. This is when I found he's been calling her during personal time, not just work hours. Also, they go out to lunches without me knowing, share coffee purchases in the morning, buying coffee for each other, etc. He insist it was an innocent friendship. But didn't tell me because I might get mad at him. He ended the friendship because I was concerned, he ended it on his own. He says they don't see each other or talk to each other anymore. She moved to a different building due to some changes. Should I still be concerned. Can I trust him?