How about this possibility: (1) You've got this lab that uses a lot of animals and does experiments. (2) There is a 'wet' market nearby that deals in animals from A to Z. (3) Maybe some animal from the lab carrying a zoonotic virus (origin unknown) somehow got disposed of for cash in the market? How could WHO or anyone uncover such an occurrence a year or more later? Would it be possible that such a thing had happened and no one ever had had any idea that it had happened?