Continuity mistake: When the old man is fixing the water fountain, a towel appears / disappears between shots.
Continuity mistake: After water falls on Billy's desk, the table and floor appear totally dry.
Continuity mistake: When Billy is tied up in the dentist's chair, he's got a single wire tying him up, but a double wire in the close-ups. The bandage on his forehead is also either covering his eyebrows or not, depending on the angle.
Answer: She only saw him briefly, never in full light either. Plus, it was some time ago. I don't think she would suspect there is another one either.
lionhead
At the end of the first movie, after killing the gremlins, everyone is at Billy's house, and Kate puts a thermometer in Gizmo's mouth and looks directly at him while doing it, giving her plenty of time to look at him.
Still, very briefly, still in low light. She has had way less interaction with Gizmo overall.
lionhead