yeah, not in the time frame available or with the tools available. It would either have to be some hidden virus that they all lied about, or somehow an unknown contaminant in their samples that then also disappeared when they looked.
All new assumptions that make this theory less likely.
Or, rather than starting with ANY virus as in your straw man, the lab started with a strain of the actual virus, from nature, and then leaked it with or without changes. Note the “or without” part. I don’t see how you rule this out.
All new assumptions that make this theory less likely.