> ...sick in November....

An acquaintance of mine was actually sick in November with COVID-like symptoms. They had a dry cough that was so nasty, doctors tested them for whooping cough (pertussis) and it was negative. And, it actually was not too long after their office got a visit from some overseas partners from China. IIRC, they even said one of the Chinese visitors had a bit of a cough at the time. I think this is pretty strong circumstantial evidence that they may have had the virus. At the very least, it's enough evidence that this person should definitely get an antibody test as soon as practical.

Edit: I initially forgot to include that the pertussis test was negative, although that was certainly implied.