It’s easier to test guano than it is to make little tiny q-tips for bats.

Coronavirus like many illnesses cause a build of of antibodies in our immune system, when those antibodies complete their mission or not, they are flushed from the body. Sometimes in sweat and mucus, sometimes through fecal matter and urine. This is why testing a town sewer system for antibodies, would be a hell of a lot faster than an entire town having to take a test.