[KennyBlanken]

Yep. Likely self-supervised isolation/quarantine and someone broke the rules.

That said, omicron is apparently insanely infectious, so it could have been something as simple as a member of the ground crew being inside the plane for a bit to stock or check something.

[avereveard]

Or some asymptomatic carrier administering a set of the test, or some of the cleaning staff coughing in the room before the tests.

That they focused on the person action (quarantines, tests) and not the environment (transfer shuttles, testing rooms, bathrooms and dressing rooms) kids hints the latter as a source. After all, you drop your mask during testing, and I've seen places doing it in a small, unventilated room with no pause between each person.