Shell Bell does in fact know what that person looks like. And since Shell Bell was on television herself not long before, albeit for only a minute, she is a little concerned that she will also be recognized. She's never met anyone else from Panem here. Maybe this is an alternate. She shouldn't be so nervous. The District Four tributes from that year weren't even people she knew; the district is big and they lived in other towns.

But why, why is Sherlock Stark watching her?

There's no point in taking down the sign and hoping to gather less attention. Sherlock has already seen it.