Yumi, of all people, was not aware of what he's been doing, despite dated him. His appearance from when he play Shogi is difference from his usual one. And it take a while for Haibara and DB to recognize him from photo. I think I will let it pass that Sera could not find him because she didn't know he play Shogi
Another thing that I'm surprise no one mention (or I never come across) is Sera call Shuuichi "Shu-ni". If her middle brother is Shukichi, will she call her big brother like that? It will be pretty confusing.
In other word. I think Sera would chose a different nickname for her big brother if two brothers's name share first syllable.