This is exactly the problem. Temperature checks would miss anyone who does not have symptoms or, in particular, this symptom. Exactly how often a coronavirus infection comes with a fever is still slightly unclear, but what is certain is that it is possible to be sick with COVID-19 and not have a fever. It’s even more likely that you wouldn’t have a fever for the duration of the infection. A case study of 5,700 patients hospitalized for COVID-19 in the New York City area published in JAMA found that just over 30 percent of patients had a fever when they came in, which caused experts to question the utility of temperature checks at all. There’s also the smaller issue that temperature checks will catch people who are running a fever for another reason (but if you have some other infection, it’s good to not be spreading that right now too).
Another problem with temperature checks: The thermal cameras that some business and airports are using to scan temperatures aren’t as accurate as individual thermometers are, meaning they’re imperfect filters even for people with fevers. Drones have the same problem.