Mishap in Isle 4.....

I found myself in the grocery store last night standing in front of the hamburger helper section with a blank look on my face because my head hurt and I was tired and I did not want to make dinner. Of course this is where two people I know found me (why not in front of the gourmet baking isle??).  The first person who noticed me is not someone I know well or have even seen recently. I couldn't even remember her name but I was pretty sure I knew where I knew her from.  She jumped right into a nice long discussion about what she had been up to and her preparations for Christmas. Thinking she was one of the mothers from scouts I asked her if she had been to the tree lot yet (we are all taking turns volunteering there to raise money). The questioning look on her face made me realize I obviously didn't know her from scouts.  I quickly re-worded my question into a very generic one - something like " Have you gotten a tree yet?" while my mind raced to place her. Finally she made a reference to football (her son played on the same team as my son. There it is!).  The conversation went pretty seamlessly from there. Ugh but that was frustrating and I felt like a fool for not fully remembering her.  I wonder how obvious it was that I couldn't place her? Probably pretty obvious. An actual light most likely went off behind my eyes giving me away.  I know there have been times when I have talked to someone and I know she did not remember who I was.  I know there have been times when I have had no clue who someone is and I'm pretty sure I pulled it off too.  There's been times when I've just had to flat out ask, "who are you?" I feel like if someone has bothered to remember who I am I should be able to give them the same courtesy. Thank goodness the second person who came upon me was someone I know fairly well so my aching brain did not have to repeat the whole process. Though, on the other hand, the first woman saw me greet the other person and probably noticed the difference between that and how I greeted her...............ugh, my head hurts again..............   
                   So, what is the protocol when someone you vaguely remember starts talking to you like you are old friends?