We know now that 50-75% of infected do not have symptoms so we were confused how they managed to track asymptomatic cases.
The virus may infect the fine capillaries in the face regardless of fever, etc.
It is very easy to identify the corona "blush": 3 rings: one sharp spot on each cheek and a pale ring of light skin around the mouth.
FLIR, military optics, etc can be used to do this long range.
I have attached some color adjusted images of the corona "blush" to help understand: https://imgur.com/a/dXz52rR
Go on New York, Florida or Seattle Instagram/Twitter and review recent seflies, isnt it strange that 20%+ of selfies in highly infected states have the 3 rings?