Noticed a diaper on another person

Today my group had a potluck in the park and it took over an hour for us to all start eating because people came late. There was an old guy there with a dog in our group and I noticed his shirt was up and there was the plastic pants and I knew he was wearing a cloth diaper. I bit my tongue but I really wanted to tell him his shirt was up. But he walked his dog and I didn’t really pay any attention to him so I kept focused on my own children and kept making sure I wasn’t showing. Then he was gone because he wasn’t there while we were eating and I don’t even know his name and I won’t recognize him in the group if I see him again. I don’t even know if he is even part of our group or if he was just a family member of our group member. But that is what you get with autism, not recognizing people and sometimes it is a good thing. But at least I can recognize my own family members and people at my work and my own husband and children so I know I do not have prosopagnosia.