Too bad you didn't get his name. Department's have policies against off-duty officers enforcing laws, except in cases of emergency. He should be held accountable, both to the law, and his department policy.
Expressing his opinion was not enforcing the law. I am sure many could make a much bigger deal out of this, but it does not seem probable you could get him in trouble for enforcing the law when nothing really happened.
No detainment, nothing to talk about really.
I have had this same thing occur to me, and while I was extremely pleasant, I enjoyed the conversation, I know the off duty officer probably did not enjoy it. He drove up in his marked cruiser, but in plain cloths. I started my recorder going when I saw him coming and never answered a single question, but I sure did ask some good ones.
He said:
You know you cant carry that around here, right?
I said:
Am I being detained officer?
Do you have a citation for the law in question you believe I am violating?
Can I give you this pamphlet that explains the law here in North Carolina?
He got real quite once he realized I was fully aware of the law. Then he said, have a nice day and drove off. I saw him a few other times, but he never bothered to stop.