There's a moment of both wariness and confusion, where she sees that gesture and her body wants to react by dodging, but her brain is telling her that Superman doesn't use guns.

But then the man speaks, and it's mostly just confusion left. He has Superman's face, but he isn't Superman.

"Right," Yup, that's a very eloquent response, as Steph squints at Solo, "Who are you then?"

It's not accusatory, despite the words; she's just curious.