Previously, when screening for diseases of sort or for radioactive material, CBP would mostly do this near where passengers gather for passport control processing or for baggage claim/customs processing. The exception may be when they have a likely known case on board a particular flight and then gate reception chances may increase. But IAD international flights mainly get deplaned by using roving moon buggies.

Resource constraints is why they tend to do this stuff at choke-points that hit larger number of passengers rather than to do this at gates, even where flights come in directly into an arrival terminal jetbridge for deplaning.

But this case at IAD this time may have been an issue of use of a dosimeter for detecting radioactive emissions than for detecting people exhibiting a fever.