I had a customer that got extremely sick in January/February, collapsed in a public place was knocked on his ass for several days, was ill and tired for a few weeks. He had no insurance so he suffered privately and was never seen. He had to test because he was contact traced as someone who might have been exposed and it was revealed that he was not currently infected but had the antibodies. There were no known local cases when he got sick but he suspects that he had COVID. I'm sure if he had thought it was COVID at the time he would have seen a doctor or let health authorities know.
I've know a few people who have tested positive on the antibody test but never felt ill besides that one customer.