You are probably talking about how previous coronavirus exposure seems to give some people some level of T-cell protection against SARS-CoV-2. T-cells are a dif

You are probably talking about how previous coronavirus exposure seems to give some people some level of T-cell protection against SARS-CoV-2. T-cells are a different part of the immune system from immunoglobulin proteins, which is what these antibody tests measure. So it's more likely that those people would show up as a negative on this test because they wouldn't have the right immunoglobulins in their blood. It's also possible that some people are fighting off COVID via a T-Cell response while not developing significant immunoglobulins or that immunoglobulin levels fade quickly while T-Cells don't. Those people can also test negative for immunoglobulin antibodies despite fighting off the disease. So it's actually more likely that these tests are significantly under counting exposed people in total. But these tests aren't perfect and for any specific person, they could give you a false positive or a false negative result.