Husband having lunch with female coworker

Interested in others' opinions -

My husband has PTO vacation this week so he is off during the day and I am at work. He mentioned maybe he would go to lunch with a woman he used to work with today, just the two of them. Not particularly strictly business but they do have the work/industry in common still. They have done it before, but more often it is in a group of coworkers. He has joked that even when they are in a group, the wait staff always think the two of them are together (like ordering/paying together). I have never attended these lunches, or met this coworker.

Is this typical husband behavior or is it suspect?