It is reported as unknown if the dog in Hong Kong that positive for covid-19 was infected (seropositive) or contaminated with the virus.
https://www.info.gov.hk/gia/general/202002/28/P2020022800013.htm
“ At present, the AFCD does not have evidence that pet animals can be infected with COVID-19 virus or can be a source of infection to people. The Department will conduct close monitoring of the above dog and collect further samples for testing to confirm if the dog has really been infected with the virus or this is a result of environmental contamination of the dog's mouth and nose. Repeated tests will be conducted for the dog and it will only be returned when the test result is negative.”
The test currently being used appears to be RT-PCR; it looks for the DNA for the virus not if there is an immune response to the presence of the virus (antibody test; seropositive or seronegative).