Statistics.

Let’s ignore the time aspect, and assume that the test has only a 35% chance of returning positive for an infected but asymptomatic patient.

Repeat that test 3 times, and the chances of at least one returning positive jumps to 73%.

(That’s why there are two tests in most rapid test boxes. You’re supposed to test twice, 24 hours apart, to improve the chances of a positive result if you are indeed infected.)