This situation somewhat happened to me the other day, funny story too.
Where I used to live, the Managers in the apartment complex were good friends of my best friends parents. We would always be at their house and they used to watch us. She would also give me haircuts all the time. They moved to Wisconsin about 10 years ago and I haven't seen them since.
Last Friday night I was working my usual night managing shift at the Sizzler. I saw a guy at the Salad Bar that I swear I recognized. After a while I saw where they were sitting and went to check up on them. After they said everything was doing great, I asked him if his name was Troy. He just sort of looked at me weird and quietly said "Yes?".
I explained to him why I asked him and he said once he noticed my name tag he recognized me. After ten years it was really odd that I recognized him. They were moving back closer to the rest of their family as he finished school and got a good position near by.
As mentioned above, he was asking me what school I went to, how my grades were, what I was doing now, what I want to do soon. We talked for a good half hour. I ended up sitting at their table talking for a while. I just asked him how his family was doing and how much their kids have grown. Of course they don't remember me, they were too young. I was interested in his schooling and his degree.