Absence of proof is not proof of absence.

Do I really need to point out to you how many animal species and potential reservoir populations there are? It's impossible to sample all of them, or even enough to be sure there is no such reservoir.

One potential explanation could be a commercial mink farm somewhere in China. The virus is very affine to mink, mink can infect Humans and Humans can infect mink. If they were the intermediate host or "reservoir" there is no way of ever proving it, because those mink are all dead now and I doubt there will be enough samples left to detect the Virus somehow.