Would it change the meaning of the context if I say "didn't" instead of "haven't"?
Native speaker’s answer
Rebecca
In this context, the meaning would not change if you say "didn't" since they are asking each other if they have seen the items they lost. If they were not asking each other, then "haven't" would be used. Ex: "Did you see my wallet lying around?" asked Kyle. "I didn't!" replied Lilly. Ex: I haven't figured out how to run this new program on my computer.