I know the faces of people close to me or those I've known a long time, but to me, total strangers will walk up and start a conversation as if they expected me to know them and I'll have to ask questions that will give me a clue. Usually former classmates or co-workers.
When I went back to the Bay Area for my sister's memorial, my daughter's mother in law was going to pick me up at the airport. Nobody told me how I was to know her or how she was to know me! I finally spotted a car of the sort I thought her son would drive, only nicer, and took a wild guess. However, she recognized me right away! HOW?!?!? We'd only seen each other once, a year ago at least!
Duh? 