Revealing mistake: The Gremlin who throws the beaker full of acid has his arm operated by a stick, which is visible as he chucks the beaker. (01:08:05)
Other mistake: Gizmo can't be exposed to sunlight, yet when the shop is demolished he walks out on the street. Note how he is walking past the shady part of the curb to the sunny part, suffering no harm whatsoever.
Continuity mistake: When Marla is trapped in the web, she either has a huge web in front of her, or nothing at all, depending on which angle is focusing.
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