I believe what happened was that immediately after Mr. Clark went missing, the person remembered she'd just seen him the day before or so, actually passed right by him. He is remarkable in that he has a disability which affects his gait. She thought it was him and even confirmed it with family members, but didn't think important to the investigation. (As an aside, I know a lot of others here think that is strange, but I don't think so. I wouldn't think that, just because I'd seen someone before they apparently disappeared, it would have anything to do with the disappearance.)
Sometime later, much later, she realized, or was told, that her seeing him immediately before he disappeared could be significant, so she told police. I really don't think it's strange at all.