More information is available from the OIE (World Organization for Animal Health) about the second dog in Hong Kong that was identified as positive for SARS-CoV-2, the virus that causes COVID-19, back on March 19th.
The dog in question, a two-year-old German Shepherd, and another dog from the same household were quarantined on March 18 when the infected owner had to be hospitalized. Both dogs were tested repeatedly by the Hong Kong authorities. The other dog was negative on all tests, but the one dog had positive test results on March 18, 19 and 20. It also later developed antibodies against the virus in its blood (as did the first dog from Hong Kong that tested positive). In this case, live virus was isolated from the dog, which provides solid confirmation that it was infected (though it’s not clear from which samples the virus was isolated).
The dog remained healthy and had negative test results after those first three positive tests. That’s consistent with the current thought that dogs have relatively low susceptibility to the virus and don’t shed it for long if they happen to become infected; however, we don’t know when the dog started shedding. We know it shed for at least 3 days once it arrived in quarantine, but we don’t know if it may have started shedding earlier than this.