ET, another possibility that should be considered is that infection had been circulating unnoticed at a low level among the population on the island for some time, possibly introduced on an object, and the travellers actually picked it up after their period of self isolation – this would also be consistent with the three otherwise unexplained cases, especially if contact tracing shows no contact with the travellers.
If there is a more widespread outbreak, waste water testing could help to narrow down the search area, to help target testing of individuals.