Same thing happened to me about 3 years ago. I was eating lunch at a "mom and pop" (i.e. did not serve alcohol) restaurant and noticed a woman in her late 30's with three kids staring at me from time to time. At one point, she got up, left the kids, and went over to talk to the guy who ran the restaurant. In about 10 minutes, a LEO enters, speaks to the owner, who is pointing his finger at me. The LEO comes over and asks me if I am armed, and I said that I was, and he asked me to "step outside" . . . all the time walking behind me with his hand on his gun. Once outside, he asked for my gun and my ID; I presented my HCP which he looked at, and gave gun and HCP back to me. He was quite nice. He reentered the restaurant, explained all to the owner, who then went over and told the woman. Thus, she was obviously the one who "turned me in."
I have still never figured out how she saw my gun. It was a cool day and I was wearing a jacket . . .