I don't understand the phrase "Now, whether we were shown everything? You can never know." Perhaps I've misunderstood the grammar, the logic of it seems to be inconsistent: Of course the speaker, the scientist knows the things he was shown to him, so you can know if he's willing to tell you. "You can never know" sounds that the speaker is determined not to tell you anything about it. But if so, why doesn't he say "I don't tell you." Besides, what if the audience try hard to make him reveal what he was shown? Such as inviting him to a party to drink - "Wine in, truth out." In that case, you can't delcare that "You can never know."
So, what should I do to understand the phrase correctly?*
Peter Ben Embarek, a food-safety and zoonosis scientist with the WHO in Geneva, Switzerland, who headed the investigation, said at the press conference that the team had conducted extensive discussions with staff at the Wuhan Institute of Virology, which has been at the centre of the speculation, and similar labs nearby. He said a leak is unlikely because the virus was not known to scientists before December 2019.
Dwyer says that the team didn’t see anything during its visits to suggest a lab accident. “Now, whether we were shown everything? You can never know. The group wasn’t designed to go and do a forensic examination of lab practice.”
*On second thought, If the phrase were written as "Now, whether we will be shown everything? You can never know.", it could be understood and the logic would be consistent because it is a future event - you don't know what a host will show you in a day in the future, do you?