Despite the ever-repetitive "look how tall you've gotten!"s and "I haven't seen you in ten years!"s from these strangers, you are required by unwritten social code to treat these people as if they were at least close enough to be a creepy uncle.
Mom: Oh you know, that guy I worked with 50 years ago, and we just happened to keep in touch. You've met him, he saw your birthing, remember?