Normally not until they (85% of the time) actually become ill. The “self-quarantine” is to provide a number of days to see if symptoms do develop. A good % remain asymptomatic and some days later test negative. But my point is that even if an asymptomatic person is out and about, provided no symptoms have started, 85% of the time, viral shedding is not yet occurring and transmission to others considered unlikely. The contagion control question is when and if someone who is asymptomatic starts to show symptoms - where are they, what are they doing, are they out, are they alone? With those questions in mind, and initial self-quarantine period is prudent, to see if symptoms do develop.
So where did the concept of the “asymptomatic spreader” come from; what research showed it?