I had a similar thing just happen to me. I was at the local supermarket picking up supplies and a nice looking blonde lady smiles and starts talking to me. I do NOT recall her from anywhere, but she sure knows me. My mind is doing process of elimination, motorcycles, school, work, the gym,.... when she finally asks if I am going to the water polo tournament tomorrow. She could probably tell I was lost, so she mentioned that her son plays on the same club team as my 13 year old.
It was one of those uncomfortable moments. How does one ask, "where do I know you from?" without coming off as being rude or clumbsy? It's like I hit age 50 and now I can not remember anything or anyone.