PCR tests for the disease which makes is very sensitive to viral loads. This is why the advice is “If you do not have symptoms but have been exposed to COVID-19, wait at least 5 full days after your exposure before taking a test.” https://www.cdc.gov/coronavirus/2019-ncov/symptoms-testing/t...

Many people will fight off an infection in those 5 days, but it’s not considered a problem as asymptomatic people are unlikely to spread the disease. PCR accuracy of asymptomatic people is still surprisingly low, but again that’s not necessarily a problem.