https://www.ncbi.nlm.nih.gov/pmc/articles/PMC416634/
I’m genuinely surprised this (and similar articles) don’t have greater citation counts.
So maybe respiratory diseases, no matter how dangerous they currently are should be heavily restricted to labs that can handle them, and then have international monitors at each of them with strict reporting rules not tied to any organization like WHO or UN or whatever, they are just there to report lab leaks no matter how small.
(on a serious note: the answer is, the scientists don't have to know about a leak for it to have happened, the important part is that they don't want to know. That is, China prefers if this question goes unanswered.)
Not saying it's true, but I don't think "they couldn't keep this a secret" is a particularly compelling rebuttal here.