"If we found it, what would we have done?"
"Let's say we found it, what would we have done?"
I don't think they are not grammatically correct, because we didn't find it. For this reason, the sentence should express a hypothetical situation. So, they should be rewritten as in
"If we had found it, what would we have done?"
"Let's say we had found it, what would we have done?"
But interestingly, after giving a survey on whether they sound natural or not, I have got a lot of votes from native speakers that they sound natural.
I guess they wouldn't regard "if we found it" or "let's say we found it" as a hypothetical situation of the past, but the real situation of the past.
I want to hear your opinions on it.