You're using the word "engineering" without defining it, and thereby conflating two things:
1. Direct artificial manipulation of a virus's genetic sequence.
2. Laboratory culture and passaging of a virus, allowing evolution due to natural mutations to proceed under artificial selective constraints.
No one serious is focusing on (1). The people bringing it up most often are either uneducated cranks, or zoonotic origin proponents using it as a strawman to refute.
The serious concern is that SARS-CoV-2 originated due to (2). In that case, since it's a natural evolutionary process, there would be no indication in the virus's genetic sequence of human tampering, because no direct human tampering occurred. The only evidence would be the absence of any animal reservoir of the virus. Perhaps that will eventually be found; but so far--unlike for SARS-CoV or MERS-CoV--it hasn't been.
As I note elsewhere in this thread, the discovery of a proximal animal host would convince me that the virus is probably of natural origin. Is there any evidence short of a direct admission from the people responsible that would convince you it might have escaped from a lab?
The WIV published lots of gain-of-function research, but all by genetic engineering and not serial passage. Ralph Baric's group has published on coronavirus gain-of-function by serial passage though:
> We adapted the SARS-CoV (Urbani strain) by serial passage in the respiratory tract of young BALB/c mice. Fifteen passages resulted in a virus (MA15) that is lethal for mice following intranasal inoculation.
So it doesn't seem implausible to me that the WIV would bring up a similar program. The lack of any public record doesn't seem like it requires any conspiracy to me, just for them not to have published yet (especially since they only got the BSL-4 lab where they'd likely be doing that work in 2018).
I think I probably understated the plausibility of a genetically-engineered origin in my comment above, too. I've just re-read Andersen's reasoning in the Nature article, and it's based heavily on the dissimilarity of SARS-CoV-2 from previously-known viruses. But we know the WIV had a private database--for example, RaTG13 was allegedly collected in 2013, but not published until after the emergence of SARS-CoV-2.
Honestly this whole area of research seems terrifying to me. Regardless of whether it's eventually shown to have caused this pandemic or not, I see no indication that this work is delivering any benefit commensurate to its risk. Many prominent epidemiologists vocally opposed the lifting of the 2018 ban on funding of gain-of-function research (e.g., Marc Lipsitch at Harvard), and I agree with them.
